In this paper, we consider the energy efficiency of implementations of video codecs for mobile devices in a top-down manner. We start from typical applications and analyse device architectures, codec implementations, and software platforms. The physical size of mobile devices limits their heat dissipation, while the battery capacity needs to be used conservingly to provide for satisfactory untethered active use time. Together with the required versatile capabilities of the devices, these are essential constraints that must be taken into account from hardware to application software design. In video decoding additional constraints come from the need to support multiple digital video coding standards, and the platform oriented design regimes of the device manufacturers.