The idea of synthesizing statecharts out of a collection of scenarios has received a lot of attention in recent years. However due to the poor expressive power of rst generation scenario languages, including UML1.x sequence diagrams, the proposed solutions often use ad hoc tricks and su er from many shortcomings. The recent adoption in UML2.0 of a richer scenario language, including interesting composition operators, now makes it possible to revisit the problem of statechart synthesis with a radically new approach. Inspired by the way UML2.0 sequence diagrams can be algebraically composed, we rst de ne an algebraic framework for composing statecharts. Then we show how to leverage the algebraic structure of UML2.0 sequence diagrams to get a direct algorithm for synthesizing a composition of statecharts out of them. The synthesized statecharts exhibit interesting properties that make them particularly useful as a basis for the detailed design process. Beyond o ering a systematic and semantically well founded method, another interest of our approach lies in its exibility: the modi cation or replacement of a g i v e n s c enario has a limited i m p act on the synthesis process, thus fostering a better traceability between the requirements and the detailed design. 1 1 This work has been partially supported by t h e F AMILIES European project. Eureka P ! 2023 Program, ITEA project ip 02009.