This paper presents a validation approach that exploits the compositional properties of a system. Our results can be applied on systems modelled by state machines and asynchronous communication by message passing. We consider two axes of composition: horizontal composition across system components, and vertical composition within components. Along the horizontal axis, we reduce the complexity of validation analysis by using a projection technique that allows us to validate interactions between components pairwise. We further simplify by introducing a set of design rules that support the development of well-formed state machines. When these rules are enforced, we are able to apply a simplified compatibility checking algorithm. Along the vertical axis, validation is applied incrementally. Elementary component collaborations are validated first, then their composite.