  Microscale energy transport processes are crucial in microelectronics, energy
harvesting devices, and emerging quantum materials. To study these processes,
methods that can probe transport with conveniently tunable length scales are
highly desirable. Transient grating spectroscopy (TGS) is such a tool that can
monitor microscale energy transport processes associated with various
fundamental energy carriers including electrons, phonons, and spins. Having
been developed and applied for a long time in the chemistry community, TGS has
regained popularity recently in studying different transport regimes in
solid-state materials. In this Tutorial, we provide an in-depth discussion of
the operational principle and instrumentation details of a modern heterodyne
TGS configuration from a practitioner's point of view. We further review recent
applications of TGS in characterizing microscale transport of heat, charge,
spin, and acoustic waves, with an emphasis on thermal transport.
