A Characterization of Lambda Definability in Categorical Models of Implicit Polymorphism Moez Alimohamed University of Pennsylvania This paper contains the work of Moez Alimohamed, a mathematics graduate student at the University of Pennsylvania who died tragically on August 29th. Lambda definability is characterized in categorical models of simply typed lambda calculus with type variables. A category-theoretic framework known as glueing or sconing is used to extend the Jung-Tiuryn characterization of lambda definability in Henkin models for the simply typed lambda calculus first to ccc models, and then to categorical models of the calculus with type variables. WWW access is http://www.cis.upenn.edu/~andre/moez.html. The paper is also available by anonymous ftp from the host ftp.cis.upenn.edu as the file pub/papers/scedrov/def.ps.Z.