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 modular header core includes a first core module including a first bore hole portion of a first bore hole, the first bore hole portion configured to couple a first electrical component with the electronic module. A second core module includes a second bore hole portion of a second bore hole different than the first bore hole, the second bore hole portion configured to couple a second electrical component with the electronic module. The first core module is detachably engaged with the second core module. A header shell is disposed around the modular header core and attached to the device container.