Asynchronous interactions are becoming more and more important in the implementation of complex B2B Web applications. This paper addresses correlation and coordination issues involved with asynchronous Web services, by studying different mechanisms and metadata structures for supporting them; in addition, several interaction patterns for building asynchronous computations are discussed, and the trade-offs between the various patterns are shown. In conclusion, we illustrate the use of asynchronous Web services in the context of some concrete B2B applications.