Abstract: Current workflow management technology does not provide adequate means for inter-workflow coordination as concurrently executing workflows are considered completely independent. While this simplified view might suffice for one application domain or the other, there are many real-world application scenarios where workflows--though independently modeled in order to remain comprehensible and manageable--are semantically interrelated. As pragmatical approaches, like merging interdependent workflows or inter-workflow message passing, do not satisfactorily solve the inter-workflow coordination problem, interaction expressions and graphs are proposed as a simple yet powerful formalism for the specification and implementation of synchronization conditions in general and inter-workflow dependencies in particular. In addition to a graph-based semi-formal interpretation of the formalism, a precise formal semantics, an equivalent operational semantics, an efficient implementation of the latter, and detailed complexity analyses have been developed allowing the formalism to be actually applied to solve real-world problems like inter-workflow coordination.