An implantable modular electrode assembly comprising: a hub comprising a plurality of electrical contacts, wherein the hub is configured to receive electrical signals from an implantable medical device; a first electrode module comprising a first substrate and a first plurality of electrodes on the first substrate, wherein the first electrode module is connectable to the hub, wherein at least some of the plurality of electrical contacts electrically communicate with the first plurality of electrodes; and a second electrode module comprising a second substrate and a second plurality of electrodes on the second substrate, wherein the second electrode module is connectable to the hub, wherein at least some of the plurality of electrical contacts electrically communicate with the second plurality of electrodes; characterized in that: the hub includes at least one rail that extends along at least a portion of a perimeter of the hub, wherein the first electrode module defines a slot that extends along at least a portion of a perimeter of the first electrode module, and wherein the 1 at least one contact of the plurality of electrical contacts of the hub is disposed on the rail; or in that the hub defines at least one slot that extends along at least a portion of a perimeter of the hub, wherein the first electrode module comprises a rail that extends along at least a portion of a perimeter of the first electrode module, and wherein the 1 at least one contact of the plurality of electrical contacts of the hub is disposed in the slot; and wherein the slot is configured to receive the rail to connect at least one electrode of the first plurality of electrodes to at least one contact of the plurality of electrical contacts of the hub. a hub comprising a plurality of electrical contacts, wherein the hub is configured to receive electrical signals from an implantable medical device; a first electrode module comprising a first substrate and a first plurality of electrodes on the first substrate, wherein the first electrode module is connectable to the hub, wherein at least some of the plurality of electrical contacts electrically communicate with the first plurality of electrodes; and a second electrode module comprising a second substrate and a second plurality of electrodes on the second substrate, wherein the second electrode module is connectable to the hub, wherein at least some of the plurality of electrical contacts electrically communicate with the second plurality of electrodes; characterized in that: the hub includes at least one rail that extends along at least a portion of a perimeter of the hub, wherein the first electrode module defines a slot that extends along at least a portion of a perimeter of the first electrode module, and wherein the 1 at least one contact of the plurality of electrical contacts of the hub is disposed on the rail; or in that the hub defines at least one slot that extends along at least a portion of a perimeter of the hub, wherein the first electrode module comprises a rail that extends along at least a portion of a perimeter of the first electrode module, and wherein the 1 at least one contact of the plurality of electrical contacts of the hub is disposed in the slot; and wherein the slot is configured to receive the rail to connect at least one electrode of the first plurality of electrodes to at least one contact of the plurality of electrical contacts of the hub. the hub includes at least one rail that extends along at least a portion of a perimeter of the hub, wherein the first electrode module defines a slot that extends along at least a portion of a perimeter of the first electrode module, and wherein the 1 at least one contact of the plurality of electrical contacts of the hub is disposed on the rail; or in that the hub defines at least one slot that extends along at least a portion of a perimeter of the hub, wherein the first electrode module comprises a rail that extends along at least a portion of a perimeter of the first electrode module, and wherein the 1 at least one contact of the plurality of electrical contacts of the hub is disposed in the slot; and wherein the slot is configured to receive the rail to connect at least one electrode of the first plurality of electrodes to at least one contact of the plurality of electrical contacts of the hub.