BPEL is currently the most widespread language for composing Web services, but it lacks formal semantics. YAWL is a workflow language with a well defined formal semantics that implements the most common workflow patterns. In this paper we provide a methodology for translating BPEL processes into YAWL workflows, thus paving the way for the formal analysis, aggregation and adaptation of BPEL processes. The approach we propose defines a YAWL pattern for each BPEL activity. The translation of a BPEL process reduces then to suitably instantiating and interconnecting the patterns of its activities.