  Thermal emission is a ubiquitous and fundamental process by which all objects
at non-zero temperatures radiate electromagnetic energy. This process is often
presented to be incoherent in both space and time, resulting in broadband,
omnidirectional light emission toward the far field, with a spectral density
related to the emitter temperature by Planck's law. Over the past two decades,
there has been considerable progress in engineering the spectrum,
directionality, polarization, and temporal response of thermally emitted light
using nanostructured materials. This review summarizes the basic physics of
thermal emission, lays out various nanophotonic approaches to engineer
thermal-emission in the far field, and highlights several relevant
applications, including energy harvesting, lighting, and radiative cooling.
