AbstractBPEL4WS (BPEL in short) is a business process definition language built natively on top of the Web services application model. BPEL provides a workflow‐oriented composition model for Web services applications, and is thus a central piece in the heavily componentized service‐oriented computing model. BPEL results from the merger of two distinct process metamodels (the process algebra model of XLANG and the graph‐oriented model of WSFL) into a coherent and powerful framework. Implementing BPEL thus presents significant challenges to middleware developers. This paper discusses those challenges and describes the design and architecture of the BPWS4J runtime, and a full implementation of the BPELWS 1.1 specification. Copyright © 2005 John Wiley & Sons, Ltd.