Abstract. Combining Higher Order Abstract Syntax (HOAS) and induction is well known to be problematic. We have implemented a tool called Hybrid, within Isabelle HOL, which does allow object logics to be represented using HOAS, and reasoned about using tactical theorem proving in general and principles of (co)induction in particular. In this paper we describe Hybrid, and illustrate its use with case studies. We also provide some theoretical adequacy results which underpin our practical work. 1 Introduction Many people are concerned with the development of computing systems which can be used to reason about and prove properties of programming languages. However, developing such systems is not easy. Difficulties abound in both practical implementation and underpinning theory. Our paper makes both a theoretical and practical contribution to this research area. More precisely, this paper concerns how to reason about object level logics with syntax involving variable binding--note that a programming language can be presented as an example of such an object logic. Our contribution is the provision of a mechanized tool, Hybrid, which has been coded within Isabelle HOL, and- provides a form of logical framework within which the syntax of an object