This paper describes the application of object-oriented programming to the design of a multiprocessor ESM testbed. The ESM testbed uses an object-oriented development environment which integrates Smalltalk and C language tools with the Harmony real-time operating system in a shared memory multiprocessor. All development for an application is done using personal computers which are themselves processors in the real-time testbed. We first discuss two aspects of the ESM testbed: a framework for investigating ESM signal processing algorithms based on an object-oriented emitter database and blackboard objects which implement probabilistic reasoning; and an object-oriented ESM simulation environment which illustrates the use of object-oriented techniques for the development of complex real-time systems. In the second part of the paper we describe our software engineering approach and tools. Throughout the paper, the role played by object-oriented programming in the design of hybrid multiprocessors is highlighted.