Heat transfer at the nanoscale can be categorized into distinct regimes, illustrated in Fig. 10.1. When the structural dimensions are approximately an order of magnitude larger than the mean free path, the heat transport is diffusive. As the size of the structure approaches or falls below the mean free path, size-related effects emerge, eventually leading to ballistic transport. These size effects are further classified into the classical size effect regime and the quantum size effect regime, depending on the ratio of the characteristic length to the carrier wavelength. These regimes are also known as the particle and wave regimes, or incoherent and coherent transport regimes. In the classical size effect regimes, the phase of the energy carriers can be disregarded, and their trajectories can be tracked instead.