Systems and methods for implantable medical devices and headers are described. In an example, an implantable medical device includes a device container including an electronic module within the device container. A header core includes an electronic connection feature electrically coupled to the electronic module within the device container, the electronic connection feature configured to engage with a lead. In some examples, the header core includes a tag holder configured to locate an identification tag in a selected position with respect to the header core. In some examples, the header core includes an antenna attachment feature configured to locate an antenna in a selected position with respect to the header core. A header shell is disposed around the header core and attached to the device container.