Computations in distributed systems can be described in terms of actions in which one or more processes synchronize by common handshakes. A general formulation for such action systems is given, together with two interleaved execution models: a serial model that allows simple temporal reasoning, and a concurrent model that reflects a distributed execution environment more faithfully. The equivalence of the two models is shown, up to fairness properties. The relationships between the natural fairness and justice notions in the two models are analyzed. This leads to sufficient conditions under which reasoning in terms of the serial model is valid even when the execution environment guarantees the weaker properties of the concurrent model only. Proving that these conditions hold for a particular system can be carried out totally within the simpler serial model. Finally, the results are discussed from the point of view of partial order computations.