We introduce a method for the specification of reactive asynchronous components with a concurrent access interface and outline its mathematical foundation. The method supports the specification of components that show a complex reactive behavior including timing aspects. Examples are the nonstrict fair merge or the arbiter. The method supports the specification of reactive systems and their modular composition into data flow networks. The specification approach is compositional. It supports the integrated specification and verification of both safety and liveness conditions in modular system descriptions. We outline particular specification styles that may be useful for the better readability of such specifications.