  The study of transient and variable events, including novae, active galactic
nuclei, and black hole binaries, has historically been a fruitful path for
elucidating the evolutionary mechanisms of our universe. The study of such
events in the millimeter and submillimeter is, however, still in its infancy.
Submillimeter observations probe a variety of materials, such as optically
thick dust, which are hard to study in other wavelengths. Submillimeter
observations are sensitive to a number of emission mechanisms, from the
aforementioned cold dust, to hot free-free emission, and synchrotron emission
from energetic particles. Study of these phenomena has been hampered by a lack
of prompt, high sensitivity submillimeter follow-up, as well as by a lack of
high-sky-coverage submillimeter surveys. In this paper, we describe how the
proposed Atacama Large Aperture Submillimeter Telescope (AtLAST) could fill in
these gaps in our understanding of the transient universe. We discuss a number
of science cases that would benefit from AtLAST observations, and detail how
AtLAST is uniquely suited to contributing to them. In particular, AtLAST's
large field of view will enable serendipitous detections of transient events,
while its anticipated ability to get on source quickly and observe
simultaneously in multiple bands make it also ideally suited for transient
follow-up. We make theoretical predictions for the instrumental and observatory
properties required to significantly contribute to these science cases, and
compare them to the projected AtLAST capabilities. Finally, we consider the
unique ways in which transient science cases constrain the observational
strategies of AtLAST, and make prescriptions for how AtLAST should observe in
order to maximize its transient science output without impinging on other
science cases.
