Activity diagrams (ADs) have recently become widely used in the modeling of workflows, business processes, and web-services, where they serve various purposes, from documentation, requirement definitions, and test case specifications, to simulation and code generation. As models, programs, and systems evolve over time, understanding changes and their impact is an important challenge, which has attracted much research efforts in recent years. In this paper we present addiff, a semantic differencing operator for ADs. Unlike most existing approaches to model comparison, which compare the concrete or the abstract syntax of two given diagrams and output a list of syntactical changes or edit operations, addiff considers the semantics of the diagrams at hand and outputs a set of diff witnesses, each of which is an execution trace that is possible in the first AD and is not possible in the second. We motivate the use of addiff, formally define it, and show two algorithms to compute it, a concrete forward-search algorithm and a symbolic fixpoint algorithm, implemented using BDDs and integrated into the Eclipse IDE. Empirical results and examples demonstrate the feasibility and unique contribution of addiff to the state-of-the-art in version comparison and evolution analysis.