  After many years of flying in space primarily for educational purposes,
CubeSats - tiny satellites with form factors corresponding to arrangements of
"1U" units, or cubes, each 10 cm on a side - have come into their own as
valuable platforms for technology advancement and scientific investigations.
CubeSats offer comparatively rapid, low-cost access to space for payloads that
be built, tested, and operated by relatively small teams, with substantial
contributions from students and early career researchers. Continuing advances
in compact, low-power detectors, readout electronics, and flight computers have
now enabled X-ray and gamma-ray sensing payloads that can fit within the
constraints of CubeSat missions, permitting in-orbit demonstrations of new
techniques and innovative high-energy astronomy observations. Gamma-ray-sensing
CubeSats are certain to make an important contribution in the new era of
multi-messenger, time-domain astronomy by detecting and localizing bright
transients such as gamma-ray bursts, solar flares, and terrestrial gamma-ray
flashes; however, other astrophysical science areas requiring long observations
in a low-background environment, including gamma-ray polarimetry, studies of
nuclear lines, and measurement of diffuse backgrounds, will likely benefit as
well. We present the primary benefits of CubeSats for high-energy astronomy,
highlight the scientific areas currently or soon to be studied, and review the
missions that are currently operating, under development, or proposed. A rich
portfolio of CubeSats for gamma-ray astronomy already exists, and the potential
for a broad range of creative and scientifically productive missions in the
near future is very high.
