Some embodiments of the invention include a thermal ground plane with a variable thickness vapor core. For example, a thermal ground plan may include a first casing and a second casing where the second casing and the first casing configured to enclose a working fluid. The thermal ground plane may also include an evaporator region disposed at least partially on at least one of the first casing and the second casing; a condenser region disposed at least partially on at least one of the first casing and the second casing; and a wicking layer disposed between the first casing and the second casing a vapor core defined at least partially by a gap between the first casing and the second casing. The thickness of the gap can vary across the first casing and the second casing.