  With the rapidly increasing integration density and power density in
nanoscale electronic devices, the thermal management concerning heat generation
and energy harvesting becomes quite crucial. Since phonon is the major heat
carrier in semiconductors, thermal transport due to phonons in mesoscopic
systems has attracted much attention. In quantum transport studies, the
nonequilibrium Green's function (NEGF) method is a versatile and powerful tool
that has been developed for several decades. In this review, we will discuss
theoretical investigations of thermal transport using the NEGF approach from
two aspects. For the aspect of phonon transport, the phonon NEGF method is
briefly introduced and its applications on thermal transport in mesoscopic
systems including one-dimensional atomic chains, multi-terminal systems, and
transient phonon transport are discussed. For the aspect of thermoelectric
transport, the caloritronic effects in which the charge, spin, and valley
degrees of freedom are manipulated by the temperature gradient are discussed.
The time-dependent thermoelectric behavior is also presented in the transient
regime within the partitioned scheme based on the NEGF method.
