In this paper we describe the Openproof heterogeneous reasoning framework. The Openproof framework provides support for the implementation of heterogeneous reasoning environments, i.e., environments for writing arguments or proofs involving a number of different kinds of representation. The resulting environments are in a similar spirit to our Hyperproof program, though the Openproof framework goes beyond Hyperproof by providing facilities for the inclusion of a variety of representation systems in the same environment. The framework serves as the core of a number of widely used educational programs including Fitch.