This paper deals with the testing of distributed systems. An implementation under test is checked for conformance with the properties defined by a reference specification. Since distributed systems usually have multiple interfaces, the reference specification will not define the order of all pairs of interactions taking place at different interfaces. Therefore a specification formalism supporting the definition of partial orders is required. Different such formalisms are compared in this paper, including MSC-Charts (or Interaction Overview Diagrams). A variation of this formalism, called Partial-Order-Charts (PO-Charts) is proposed which makes abstraction from the exchange of messages. It concentrates on the specification of partial orders between local actions in different system components. It is shown that the partial-order testing approach introduced for a single partial order specification can be adapted to testing PO-Charts which define various combinations of different partial orders which are sequenced by strict or week sequencing, including loops. Various examples are given to compare this testing approach with state machine testing methods which can be applied for bounded PO-Charts for which one can derive an equivalent state machine. The testing complexities and fault model assumptions of these two approaches are compared.