diff --git a/transformer_lens/components/mlps/can_be_used_as_mlp.py b/transformer_lens/components/mlps/can_be_used_as_mlp.py
index b0945276b..6ff660e0c 100644
--- a/transformer_lens/components/mlps/can_be_used_as_mlp.py
+++ b/transformer_lens/components/mlps/can_be_used_as_mlp.py
@@ -17,6 +17,7 @@
 from transformer_lens.hook_points import HookPoint
 from transformer_lens.HookedTransformerConfig import HookedTransformerConfig
 from transformer_lens.utilities.activation_functions import ActivationFunction
+from transformer_lens.utils import XIELU
 
 
 class CanBeUsedAsMLP(nn.Module):
@@ -65,7 +66,10 @@ def select_activation_function(self) -> None:
             ValueError: If the configure activation function is not supported.
         """
 
-        self.act_fn = ActivationFunctionFactory.pick_activation_function(self.cfg)
+        if self.cfg.act_fn == "xielu":
+            self.act_fn = XIELU()
+        else:
+            self.act_fn = ActivationFunctionFactory.pick_activation_function(self.cfg)
 
         if self.cfg.is_layer_norm_activation():
             self.hook_mid = HookPoint()
diff --git a/transformer_lens/loading_from_pretrained.py b/transformer_lens/loading_from_pretrained.py
index 8bfb6315d..0a54c3304 100644
--- a/transformer_lens/loading_from_pretrained.py
+++ b/transformer_lens/loading_from_pretrained.py
@@ -24,6 +24,7 @@
 import transformer_lens.utils as utils
 from transformer_lens.HookedTransformerConfig import HookedTransformerConfig
 from transformer_lens.pretrained.weight_conversions import (
+    convert_apertus_weights,
     convert_bert_weights,
     convert_bloom_weights,
     convert_coder_weights,
@@ -245,6 +246,8 @@
     "microsoft/phi-2",
     "microsoft/Phi-3-mini-4k-instruct",
     "microsoft/phi-4",
+    "swiss-ai/Apertus-8B-2509",
+    "swiss-ai/Apertus-8B-Instruct-2509",
     "google/gemma-2b",
     "google/gemma-7b",
     "google/gemma-2b-it",
@@ -701,6 +704,8 @@
     "microsoft/phi-2": ["phi-2"],
     "microsoft/Phi-3-mini-4k-instruct": ["phi-3"],
     "microsoft/phi-4": ["phi-4"],
+    "swiss-ai/Apertus-8B-2509": ["apertus-8b", "apertus"],
+    "swiss-ai/Apertus-8B-Instruct-2509": ["apertus-8b-instruct", "apertus-instruct"],
     "google/gemma-2b": ["gemma-2b"],
     "google/gemma-7b": ["gemma-7b"],
     "google/gemma-2b-it": ["gemma-2b-it"],
@@ -742,6 +747,7 @@
     "microsoft/phi-2",
     "microsoft/Phi-3-mini-4k-instruct",
     "microsoft/phi-4",
+    "swiss-ai/Apertus-",
 )
 
 
@@ -1436,6 +1442,43 @@ def convert_hf_model_config(model_name: str, **kwargs: Any):
             "parallel_attn_mlp": False,
             "rotary_dim": hf_config.hidden_size // hf_config.num_attention_heads,
         }
+    elif architecture == "ApertusForCausalLM":
+        n_heads = hf_config.num_attention_heads
+        d_head = hf_config.hidden_size // n_heads
+        num_kv_heads = getattr(hf_config, "num_key_value_heads", n_heads)
+        n_kv_heads = num_kv_heads if num_kv_heads != n_heads else None
+        cfg_dict = {
+            "d_model": hf_config.hidden_size,
+            "d_head": d_head,
+            "n_heads": n_heads,
+            "n_key_value_heads": n_kv_heads,
+            "d_mlp": hf_config.intermediate_size,
+            "n_layers": hf_config.num_hidden_layers,
+            "n_ctx": hf_config.max_position_embeddings,
+            "eps": hf_config.rms_norm_eps,
+            "d_vocab": hf_config.vocab_size,
+            "act_fn": hf_config.hidden_act,
+            "normalization_type": "RMS",
+            "positional_embedding_type": "rotary",
+            "rotary_dim": d_head,
+            "rotary_base": getattr(hf_config, "rope_theta", None),
+            "gated_mlp": False,
+            "final_rms": True,
+            "use_qk_norm": getattr(hf_config, "qk_norm", False),
+        }
+        rope_scaling = getattr(hf_config, "rope_scaling", None)
+        if rope_scaling:
+            rope_type = (rope_scaling.get("type") or rope_scaling.get("rope_type") or "").lower()
+        else:
+            rope_type = ""
+        if rope_type == "llama3":
+            cfg_dict["use_NTK_by_parts_rope"] = True
+            cfg_dict["NTK_original_ctx_len"] = rope_scaling.get(
+                "original_max_position_embeddings", hf_config.max_position_embeddings
+            )
+            cfg_dict["NTK_by_parts_low_freq_factor"] = rope_scaling.get("low_freq_factor", 1.0)
+            cfg_dict["NTK_by_parts_high_freq_factor"] = rope_scaling.get("high_freq_factor", 4.0)
+            cfg_dict["NTK_by_parts_factor"] = rope_scaling.get("factor", 1.0)
 
     elif official_model_name.startswith("google/gemma-2b"):
         # Architecture for Gemma 2b and Gemma 2b Instruct models
@@ -1986,6 +2029,8 @@ def get_pretrained_state_dict(
             state_dict = convert_gemma_weights(hf_model, cfg)
         elif cfg.original_architecture == "Gemma2ForCausalLM":
             state_dict = convert_gemma_weights(hf_model, cfg)
+        elif cfg.original_architecture == "ApertusForCausalLM":
+            state_dict = convert_apertus_weights(hf_model, cfg)
         else:
             raise ValueError(
                 f"Loading weights from the architecture is not currently supported: {cfg.original_architecture}, generated from model name {cfg.model_name}. Feel free to open an issue on GitHub to request this feature."
diff --git a/transformer_lens/pretrained/weight_conversions/__init__.py b/transformer_lens/pretrained/weight_conversions/__init__.py
index c5ea9581b..b0defcf4c 100644
--- a/transformer_lens/pretrained/weight_conversions/__init__.py
+++ b/transformer_lens/pretrained/weight_conversions/__init__.py
@@ -19,3 +19,4 @@
 from .nanogpt import convert_nanogpt_weights
 from .t5 import convert_t5_weights
 from .neel_solu_old import convert_neel_solu_old_weights
+from .apertus import convert_apertus_weights
diff --git a/transformer_lens/pretrained/weight_conversions/apertus.py b/transformer_lens/pretrained/weight_conversions/apertus.py
new file mode 100644
index 000000000..739a84d83
--- /dev/null
+++ b/transformer_lens/pretrained/weight_conversions/apertus.py
@@ -0,0 +1,123 @@
+"""
+Apertus is Llama like model architecture from Swiss AI.
+convert weights to standardized format for HookedTransformer
+"""
+
+from typing import cast
+
+import einops
+import torch
+
+from transformer_lens.HookedTransformerConfig import HookedTransformerConfig
+
+
+def convert_apertus_weights(apertus, cfg: HookedTransformerConfig):
+    state_dict = {}
+
+    state_dict["embed.W_E"] = apertus.model.embed_tokens.weight
+
+    using_gqa = cfg.n_key_value_heads is not None
+    gqa_uscore = "_" if using_gqa else ""
+
+    n_kv_heads = cast(int, cfg.n_key_value_heads if using_gqa else cfg.n_heads)
+
+
+    assert cfg.d_mlp is not None  # keep mypy happy
+
+    for l in range(cfg.n_layers):
+        state_dict[f"blocks.{l}.ln1.w"] = apertus.model.layers[l].attention_layernorm.weight
+        state_dict[f"blocks.{l}.ln1.b"] = torch.zeros(cfg.d_model, dtype=cfg.dtype, device=cfg.device)
+
+        W_Q = apertus.model.layers[l].self_attn.q_proj.weight
+        W_K = apertus.model.layers[l].self_attn.k_proj.weight
+        W_V = apertus.model.layers[l].self_attn.v_proj.weight
+
+        # in case of quantization,
+        # parameters should stay as bitsandbytes.nn.modules.Params4bit
+        if not cfg.load_in_4bit:
+            W_Q = einops.rearrange(W_Q, "(n h) m->n m h", n=cfg.n_heads)
+            W_K = einops.rearrange(W_K, "(n h) m->n m h", n=n_kv_heads)
+            W_V = einops.rearrange(W_V, "(n h) m->n m h", n=n_kv_heads)
+
+        state_dict[f"blocks.{l}.attn.W_Q"] = W_Q
+        state_dict[f"blocks.{l}.attn.{gqa_uscore}W_K"] = W_K
+        state_dict[f"blocks.{l}.attn.{gqa_uscore}W_V"] = W_V
+
+        state_dict[f"blocks.{l}.attn.b_Q"] = torch.zeros(
+            cfg.n_heads, cfg.d_head, dtype=cfg.dtype, device=cfg.device
+        )
+        state_dict[f"blocks.{l}.attn.{gqa_uscore}b_K"] = torch.zeros(
+            n_kv_heads,
+            cfg.d_head,
+            dtype=cfg.dtype,
+            device=cfg.device,
+        )
+        state_dict[f"blocks.{l}.attn.{gqa_uscore}b_V"] = torch.zeros(
+            n_kv_heads,
+            cfg.d_head,
+            dtype=cfg.dtype,
+            device=cfg.device,
+        )
+
+        W_O = apertus.model.layers[l].self_attn.o_proj.weight
+
+        if not cfg.load_in_4bit:
+            W_O = einops.rearrange(W_O, "m (n h)->n h m", n=cfg.n_heads)
+
+        state_dict[f"blocks.{l}.attn.W_O"] = W_O.to(device=cfg.device)
+
+        state_dict[f"blocks.{l}.attn.b_O"] = torch.zeros(
+            cfg.d_model, dtype=cfg.dtype, device=cfg.device
+        )
+
+        state_dict[f"blocks.{l}.ln2.w"] = apertus.model.layers[l].feedforward_layernorm.weight
+        state_dict[f"blocks.{l}.ln2.b"] = torch.zeros(cfg.d_model, dtype=cfg.dtype, device=cfg.device)
+
+        # in case of quantization,
+        # parameters should stay as bitsandbytes.nn.modules.Params4bit
+        if not cfg.load_in_4bit:
+            state_dict[f"blocks.{l}.mlp.W_in"] = apertus.model.layers[l].mlp.up_proj.weight.T
+            state_dict[f"blocks.{l}.mlp.W_out"] = apertus.model.layers[l].mlp.down_proj.weight.T
+        else:
+            state_dict[f"blocks.{l}.mlp.W_in"] = apertus.model.layers[l].mlp.up_proj.weight
+            state_dict[f"blocks.{l}.mlp.W_out"] = apertus.model.layers[l].mlp.down_proj.weight
+
+        state_dict[f"blocks.{l}.mlp.b_in"] = torch.zeros(
+            cfg.d_mlp, dtype=cfg.dtype, device=cfg.device
+        )
+        state_dict[f"blocks.{l}.mlp.b_out"] = torch.zeros(
+            cfg.d_model, dtype=cfg.dtype, device=cfg.device
+        )
+
+        # Extract trainable activation parameters
+        mlp = apertus.model.layers[l].mlp
+        try:
+            if hasattr(mlp, 'act_fn'):
+                alpha_p = mlp.act_fn.alpha_p
+                alpha_n = mlp.act_fn.alpha_n
+                beta = mlp.act_fn.beta
+            elif hasattr(mlp, 'act'):
+                alpha_p = mlp.act.alpha_p
+                alpha_n = mlp.act.alpha_n
+                beta = mlp.act.beta
+            else:
+                alpha_p = mlp.alpha_p
+                alpha_n = mlp.alpha_n
+                beta = mlp.beta
+            state_dict[f"blocks.{l}.mlp.act_fn.alpha_p"] = alpha_p
+            state_dict[f"blocks.{l}.mlp.act_fn.alpha_n"] = alpha_n
+            state_dict[f"blocks.{l}.mlp.act_fn.beta"] = beta
+        except AttributeError:
+            # If parameters not found, use defaults
+            print(f"Activation parameters not found in layer {l}, using defaults")
+            state_dict[f"blocks.{l}.mlp.act_fn.alpha_p"] = torch.tensor(0.8, dtype=cfg.dtype, device=cfg.device)
+            state_dict[f"blocks.{l}.mlp.act_fn.alpha_n"] = torch.tensor(0.8, dtype=cfg.dtype, device=cfg.device)
+            state_dict[f"blocks.{l}.mlp.act_fn.beta"] = torch.tensor(0.5, dtype=cfg.dtype, device=cfg.device)
+
+    state_dict["ln_final.w"] = apertus.model.norm.weight
+    state_dict["ln_final.b"] = torch.zeros(cfg.d_model, dtype=cfg.dtype, device=cfg.device)
+
+    state_dict["unembed.W_U"] = apertus.lm_head.weight.T
+    state_dict["unembed.b_U"] = torch.zeros(cfg.d_vocab, dtype=cfg.dtype, device=cfg.device)
+
+    return state_dict
diff --git a/transformer_lens/utilities/activation_functions.py b/transformer_lens/utilities/activation_functions.py
index 6cc701360..9cfa6eeb9 100644
--- a/transformer_lens/utilities/activation_functions.py
+++ b/transformer_lens/utilities/activation_functions.py
@@ -7,7 +7,7 @@
 import torch
 import torch.nn.functional as F
 
-from transformer_lens.utils import gelu_fast, gelu_new, solu
+from transformer_lens.utils import gelu_fast, gelu_new, solu, xielu
 
 # Convenient type for the format of each activation function
 ActivationFunction = Callable[..., torch.Tensor]
@@ -23,4 +23,5 @@
     "relu": F.relu,
     "gelu": F.gelu,
     "gelu_pytorch_tanh": lambda tensor: F.gelu(tensor, approximate="tanh"),
+    "xielu": xielu,
 }
diff --git a/transformer_lens/utils.py b/transformer_lens/utils.py
index c0992848a..67c5c6652 100644
--- a/transformer_lens/utils.py
+++ b/transformer_lens/utils.py
@@ -30,6 +30,7 @@
 
 from transformer_lens.FactoredMatrix import FactoredMatrix
 
+
 CACHE_DIR = constants.HUGGINGFACE_HUB_CACHE
 USE_DEFAULT_VALUE = None
 
@@ -203,6 +204,63 @@ def solu(input: Float[torch.Tensor, "batch pos d_mlp"]) -> Float[torch.Tensor, "
     return input * F.softmax(input, dim=-1)
 
 
+class XIELU(nn.Module):
+    """
+    Trainable xIELU activation function as described by
+    https://arxiv.org/abs/2411.13010
+
+    Defined as:
+    f(x) = {
+        α_p * x² + β * x,                                    if x > 0
+        α_n * (exp(min(x, ε)) - 1) - α_n * x + β * x,       if x ≤ 0
+    }
+    where α_p, α_n, β are trainable parameters.
+    """
+    def __init__(self, alpha_p_init: float = 0.8, alpha_n_init: float = 0.8, beta_init: float = 0.5, eps: float = -1e-6):
+        super().__init__()
+        self.alpha_p = nn.Parameter(torch.tensor(alpha_p_init, dtype=torch.float32))
+        self.alpha_n = nn.Parameter(torch.tensor(alpha_n_init, dtype=torch.float32))
+        self.beta = nn.Parameter(torch.tensor(beta_init, dtype=torch.float32))
+        self.eps = eps
+
+    def forward(self, input: Float[torch.Tensor, "batch pos d_mlp"]) -> Float[torch.Tensor, "batch pos d_mlp"]:
+        return torch.where(
+            input > 0,
+            self.alpha_p * input ** 2 + self.beta * input,
+            self.alpha_n * torch.expm1(torch.clamp_max(input, self.eps)) - self.alpha_n * input + self.beta * input
+        )
+
+
+def xielu(
+    input: Float[torch.Tensor, "batch pos d_mlp"]
+) -> Float[torch.Tensor, "batch pos d_mlp"]:
+    """
+    xIELU activation function as described by
+    https://arxiv.org/abs/2411.13010
+
+    and original code in:
+    https://github.com/rubber-duck-debug/xielu 
+
+    Defined as 
+
+    f(x) = {
+        α_p * x² + β * x,                                    if x > 0
+        α_n * (exp(min(x, ε)) - 1) - α_n * x + β * x,       if x ≤ 0
+        }
+
+    in this function the values are FIXED. However, the script can_be_used_as_mlp.py correctly used the XIELU class with trainable parameters, so the parameters can be trained if desired.
+    """
+    alpha_p: float = 0.8
+    alpha_n: float = 0.8
+    beta: float = 0.5
+    eps: float = -1e-6
+    
+    # The core calculation logic:
+    return torch.where(input > 0,
+                       alpha_p * input * input + beta * input,
+                       alpha_n * torch.expm1(torch.clamp_max(input, eps)) - alpha_n * input + beta * input)
+
+
 ACTIVATION_FN_DICT = {
     "solu": solu,
     "solu_ln": solu,
@@ -212,6 +270,7 @@ def solu(input: Float[torch.Tensor, "batch pos d_mlp"]) -> Float[torch.Tensor, "
     "relu": F.relu,
     "gelu": F.gelu,
     "gelu_pytorch_tanh": gelu_pytorch_tanh,
+    "xielu": xielu,
 }