diff --git a/src/lean_spec/__main__.py b/src/lean_spec/__main__.py
index 378d5663..dcde40ac 100644
--- a/src/lean_spec/__main__.py
+++ b/src/lean_spec/__main__.py
@@ -9,6 +9,7 @@
     python -m lean_spec --genesis config.yaml --bootnode enr:-IS4QHCYrYZbAKW...
     python -m lean_spec --genesis config.yaml --checkpoint-sync-url http://localhost:5052
     python -m lean_spec --genesis config.yaml --validator-keys ./keys --node-id lean_spec_0
+    python -m lean_spec --genesis config.yaml --validator-keys ./keys --is-aggregator
 
 Options:
     --genesis              Path to genesis YAML file (required)
@@ -17,6 +18,7 @@
     --checkpoint-sync-url  URL to fetch finalized checkpoint state for fast sync
     --validator-keys       Path to validator keys directory
     --node-id              Node identifier for validator assignment (default: lean_spec_0)
+    --is-aggregator        Enable aggregator mode for attestation aggregation (default: false)
 """
 
 from __future__ import annotations
@@ -163,6 +165,7 @@ def _init_from_genesis(
     genesis: GenesisConfig,
     event_source: LiveNetworkEventSource,
     validator_registry: ValidatorRegistry | None = None,
+    is_aggregator: bool = False,
 ) -> Node:
     """
     Initialize a node from genesis configuration.
@@ -171,6 +174,7 @@ def _init_from_genesis(
         genesis: Genesis configuration with time and validators.
         event_source: Network transport for the node.
         validator_registry: Optional registry with validator secret keys.
+        is_aggregator: Enable aggregator mode for attestation aggregation.
 
     Returns:
         A fully initialized Node starting from genesis.
@@ -192,6 +196,7 @@ def _init_from_genesis(
         network=event_source.reqresp_client,
         validator_registry=validator_registry,
         fork_digest=GOSSIP_FORK_DIGEST,
+        is_aggregator=is_aggregator,
     )
 
     # Create and return the node.
@@ -203,6 +208,7 @@ async def _init_from_checkpoint(
     genesis: GenesisConfig,
     event_source: LiveNetworkEventSource,
     validator_registry: ValidatorRegistry | None = None,
+    is_aggregator: bool = False,
 ) -> Node | None:
     """
     Initialize a node from a checkpoint state fetched from a remote node.
@@ -229,6 +235,7 @@ async def _init_from_checkpoint(
         genesis: Local genesis configuration for validation.
         event_source: Network transport for the node.
         validator_registry: Optional registry with validator secret keys.
+        is_aggregator: Enable aggregator mode for attestation aggregation.
 
     Returns:
         A fully initialized Node if successful, None if checkpoint sync failed.
@@ -301,6 +308,7 @@ async def _init_from_checkpoint(
             network=event_source.reqresp_client,
             validator_registry=validator_registry,
             fork_digest=GOSSIP_FORK_DIGEST,
+            is_aggregator=is_aggregator,
         )
 
         # Create node and inject checkpoint store.
@@ -392,6 +400,7 @@ async def run_node(
     validator_keys_path: Path | None = None,
     node_id: str = "lean_spec_0",
     genesis_time_now: bool = False,
+    is_aggregator: bool = False,
 ) -> None:
     """
     Run the lean consensus node.
@@ -404,6 +413,7 @@ async def run_node(
         validator_keys_path: Optional path to validator keys directory.
         node_id: Node identifier for validator assignment.
         genesis_time_now: Override genesis time to current time for testing.
+        is_aggregator: Enable aggregator mode for attestation aggregation.
     """
     import time
 
@@ -431,6 +441,10 @@ async def run_node(
         len(genesis.genesis_validators),
     )
 
+    # Log aggregator mode if enabled
+    if is_aggregator:
+        logger.info("Aggregator mode enabled - node will perform attestation aggregation")
+
     # Load validator keys if path provided.
     #
     # The registry holds secret keys for validators assigned to this node.
@@ -504,6 +518,7 @@ async def run_node(
             genesis=genesis,
             event_source=event_source,
             validator_registry=validator_registry,
+            is_aggregator=is_aggregator,
         )
         if node is None:
             # Checkpoint sync failed. Exit rather than falling back.
@@ -516,6 +531,7 @@ async def run_node(
             genesis=genesis,
             event_source=event_source,
             validator_registry=validator_registry,
+            is_aggregator=is_aggregator,
         )
 
     logger.info("Node initialized, peer_id=%s", event_source.connection_manager.peer_id)
@@ -646,6 +662,11 @@ def main() -> None:
         action="store_true",
         help="Override genesis time to current time (for testing)",
     )
+    parser.add_argument(
+        "--is-aggregator",
+        action="store_true",
+        help="Enable aggregator mode (node performs attestation aggregation)",
+    )
 
     args = parser.parse_args()
 
@@ -663,6 +684,7 @@ def main() -> None:
                 args.validator_keys,
                 args.node_id,
                 args.genesis_time_now,
+                args.is_aggregator,
             )
         )
     except KeyboardInterrupt:
diff --git a/src/lean_spec/subspecs/networking/enr/enr.py b/src/lean_spec/subspecs/networking/enr/enr.py
index 1f2f3e24..81d38b26 100644
--- a/src/lean_spec/subspecs/networking/enr/enr.py
+++ b/src/lean_spec/subspecs/networking/enr/enr.py
@@ -186,6 +186,14 @@ def sync_committee_subnets(self) -> SyncCommitteeSubnets | None:
             return SyncCommitteeSubnets.decode_bytes(syncnets)
         return None
 
+    @property
+    def is_aggregator(self) -> bool:
+        """Check if node advertises aggregator capability."""
+        aggregator_bytes = self.get(keys.IS_AGGREGATOR)
+        if aggregator_bytes and len(aggregator_bytes) == 1:
+            return aggregator_bytes[0] == 0x01
+        return False
+
     def is_valid(self) -> bool:
         """
         Check structural validity (does NOT verify cryptographic signature).
diff --git a/src/lean_spec/subspecs/networking/enr/keys.py b/src/lean_spec/subspecs/networking/enr/keys.py
index f45bf719..a39f09a9 100644
--- a/src/lean_spec/subspecs/networking/enr/keys.py
+++ b/src/lean_spec/subspecs/networking/enr/keys.py
@@ -47,3 +47,6 @@
 
 SYNCNETS: Final[EnrKey] = "syncnets"
 """Sync committee subnet subscriptions (1 byte bitvector)."""
+
+IS_AGGREGATOR: Final[EnrKey] = "is_aggregator"
+"""Aggregator capability flag (1 byte: 0x00 = false, 0x01 = true)."""
diff --git a/src/lean_spec/subspecs/networking/service/service.py b/src/lean_spec/subspecs/networking/service/service.py
index e949643e..193c64f1 100644
--- a/src/lean_spec/subspecs/networking/service/service.py
+++ b/src/lean_spec/subspecs/networking/service/service.py
@@ -75,6 +75,9 @@ class NetworkService:
     fork_digest: str = field(default="0x00000000")
     """Fork digest for gossip topics (4-byte hex string)."""
 
+    is_aggregator: bool = field(default=False)
+    """Whether this node functions as an aggregator."""
+
     _running: bool = field(default=False, repr=False)
     """Whether the event loop is running."""
 
diff --git a/src/lean_spec/subspecs/node/helpers.py b/src/lean_spec/subspecs/node/helpers.py
index f1cdf7f7..8ffe6a83 100644
--- a/src/lean_spec/subspecs/node/helpers.py
+++ b/src/lean_spec/subspecs/node/helpers.py
@@ -3,18 +3,24 @@
 from lean_spec.subspecs.containers.validator import ValidatorIndex
 
 
-def is_aggregator(validator_id: ValidatorIndex | None) -> bool:
+def is_aggregator(
+    validator_id: ValidatorIndex | None,
+    node_is_aggregator: bool = False,
+) -> bool:
     """
     Determine if a validator is an aggregator.
 
+    A validator acts as an aggregator when:
+    1. The validator is active (validator_id is not None)
+    2. The node operator has enabled aggregator mode
+
     Args:
         validator_id: The index of the validator.
+        node_is_aggregator: Whether the node is configured as an aggregator.
 
     Returns:
-        True if the validator is an aggregator, False otherwise.
+        True if the validator should perform aggregation, False otherwise.
     """
     if validator_id is None:
         return False
-    return (
-        False  # Placeholder implementation, in future should be defined by node operator settings
-    )
+    return node_is_aggregator
diff --git a/src/lean_spec/subspecs/node/node.py b/src/lean_spec/subspecs/node/node.py
index c4ec515c..1b612d7a 100644
--- a/src/lean_spec/subspecs/node/node.py
+++ b/src/lean_spec/subspecs/node/node.py
@@ -94,6 +94,18 @@ class NodeConfig:
     For devnet testing with ream, use "devnet0".
     """
 
+    is_aggregator: bool = field(default=False)
+    """
+    Whether this node functions as an aggregator.
+
+    When True:
+    - The node performs attestation aggregation operations
+    - The ENR advertises aggregator capability to peers
+
+    When False (default):
+    - The node runs in standard validator or passive mode
+    """
+
 
 def get_local_validator_id(registry: ValidatorRegistry | None) -> ValidatorIndex | None:
     """
@@ -218,6 +230,7 @@ def from_genesis(cls, config: NodeConfig) -> Node:
             clock=clock,
             network=config.network,
             database=database,
+            is_aggregator=config.is_aggregator,
         )
 
         chain_service = ChainService(sync_service=sync_service, clock=clock)
@@ -225,6 +238,7 @@ def from_genesis(cls, config: NodeConfig) -> Node:
             sync_service=sync_service,
             event_source=config.event_source,
             fork_digest=config.fork_digest,
+            is_aggregator=config.is_aggregator,
         )
 
         # Create API server if configured
diff --git a/src/lean_spec/subspecs/sync/service.py b/src/lean_spec/subspecs/sync/service.py
index 37a4d5a3..e210d16f 100644
--- a/src/lean_spec/subspecs/sync/service.py
+++ b/src/lean_spec/subspecs/sync/service.py
@@ -150,6 +150,9 @@ class SyncService:
     database: Database | None = field(default=None)
     """Optional database for persisting blocks and states."""
 
+    is_aggregator: bool = field(default=False)
+    """Whether this node functions as an aggregator."""
+
     process_block: BlockProcessor = field(default=default_block_processor)
     """Block processor function. Defaults to Store.on_block()."""
 
@@ -425,7 +428,10 @@ async def on_gossip_attestation(
         from lean_spec.subspecs.node.helpers import is_aggregator
 
         # Check if we are an aggregator
-        is_aggregator_role = is_aggregator(self.store.validator_id)
+        is_aggregator_role = is_aggregator(
+            self.store.validator_id,
+            node_is_aggregator=self.is_aggregator,
+        )
 
         # Integrate the attestation into forkchoice state.
         #