  I review the theoretical expectactions for the top-quark mass in a variety of
models: the Standard Model, unified models (GUTs), low-energy supersymmetric
models (SUSY), unified supersymmetric models (SUSY GUTs), supergravity models,
and superstring models. In all instances I consider the constraints on the
top-quark mass which arise by demanding that these theories be weakly
interacting. This assumption is quantified by the use of partial-wave unitarity
or triviality. The resulting upper bounds on the top-quark mass are most
stringent in SUSY GUTs models ($m^{\rm pole}_t\lsim200\sin\beta\GeV$). I also
discuss a class of $SU(5)\times U(1)$ superstring models where $m^{\rm
pole}_t\sim(170-195)\GeV$ is predicted. I conclude that experimental
determinations of the top-quark mass can be {\em naturally} understood in SUSY
GUTs and superstring models. (Lecture presented at the International School of
Subnuclear Physics, 32nd Course: From Superstring to Present-Day Physics,
Erice--Sicily: July 3--11, 1994.)

High Energy Physics - Phenomenology