Abstract: In this paper we describe how object-oriented concepts can be exploited in the design of modular robot systems. The approach is motivated in particular by the observation that when sensors or end-effectors are mounted on mobile bases or at the end of manipulator arms, they can be said to acquire, or 'inherit', motion functionality. Other aspects of modular robotics systems similarly afford parallels with other object-oriented concepts, including polymorphism, aggregation and encapsulation or abstraction. The key contribution of the model we describe is the way in which it exploits qualitative descriptions of modules to reason about modules and their relationships in modular robot configurations. The model proposed is still in its infancy, but offers benefits directly to modular robot system design (reducing software development efforts) and to modular object systems in general.