Implantable medical devices (IMDs) may be configured to provide one or more therapies to a patient.  For example, an IMD may be implantable within the body of a patient to deliver electrical stimulation therapy such as cardiac stimulation therapy or neurostimulation therapy.  An example of cardiac stimulation therapy is cardiac pacing, which may include bradycardia pacing, antitachycardia pacing, or cardiac resynchronization therapy.  An IMD may that delivers cardiac stimulation therapy may also provide cardioversion or defibrillation.  Examples of neurostimulation therapy include spinal cord stimulation, deep brain stimulation, gastric stimulation, peripheral nerve stimulation, or pelvic floor stimulation.  In other examples, an IMD may be configured to deliver drug therapy to a patient. 
In addition to, or instead of delivering therapy, an IMD may be configured to sense one or more physiological parameters of a patient.  For example, an IMD may be configured to sense various electrical signals of a patient, such as a cardiac electrogram signal, an electroencephalogram or other brain signal, or an electromyogram signal.  As other examples, an IMD may be configured to sense a cardiovascular or cerebral spinal fluid pressure or flow, heart sounds, patient movement or posture, temperature, blood oxygen saturation, respiration, edema, or pH.
In some examples, an IMD may include a hermetically sealed housing that encloses internal circuitry such as a hybrid circuit board and one or more batteries.  The IMD may also include a header portion, referred to as a header, which may include an insulating block configured to isolate one or more conductors from each other and the surrounding environment.  The header portion may be configured to house one or more components of the IMD, such as an antenna or electrode.