Thermal ground planes, thermal ground plane structures, and methods of thermal energy management, are disclosed. Embodiments of the disclosure provide for thermal ground planes and/or thermal ground plane structures for both fully passive and active two-phase heat spreaders, as well as the cooling devices and thermal management methods enabled by these planes and/or structures that enable heat dissipation from one or more high heat flux domains followed by spreading the heat laterally over a large area achieving essentially the spatially isothermal plane, structure, system, or device.