Current workflow management technology does not provide adequate means for describing and implementing workflow ensembles, i. e., dynamically evolving collections of more or less independent workflows which have to synchronize only now and then. Interaction expressions are proposed as a
simple yet powerful formalism to remedy this shortcoming. Besides operators for sequential composition, iteration, and selection, which are well-known from regular expressions, they provide parallel composition and iteration, conjunction, and several advanced features like parametric
expressions, multipliers, and quantifiers. The paper introduces interaction expressions semi-formally,
gives examples of their typical use, and describes their implementation and integration with state-of-the-art workflow technology. Major design principles, such as orthogonality and implicit and predictive choice, are discussed and compared with several related approaches
