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 t <∼ 200 sin βGeV). I also discuss a class of SU(5) × U(1) superstring models where m t ∼ (170 − 195)GeV is predicted. I conclude that experimental determinations of the top-quark mass can be 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.)