Theoretical expectations for the top-quark mass

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.)