An implantable medical device comprises one or more electrical stimulation generators, and a housing that contains the one or more electrical stimulation generators. The implantable medical device also includes a first medical lead no greater than about 6 inches in length, and a second medical lead no greater than about 6 inches in length. The housing includes a first connector block that electrically connects the first medical lead to at least one of the one or more electrical stimulation generators, and a second connector block that electrically connects the second medical lead to at least one of the one or more electrical stimulation generators. The implantable medical device may be part of an electrical stimulation system implanted beneath the skin and inferior to the inion of a patient to deliver stimulation therapy to at least one of an occipital nerve and a branch of the occipital nerve.