The AltaRica formalism is designed for describing complex systems consisting of a number of interacting components. Its semantics is expressed in terms of transition systems so that a system described in this language can be analysed by any technique or tool applicable to transition systems. The components of a system have two kinds of interactions • event synchronisation, like in the synchronized product of transition systems of Arnold and Nivat, • interface coordination: with each component are associated interfaces whose values depend on the state of the component as well as on the values of interfaces of other components of the system. Another feature of AltaRica is the possibility of defining hierarchical systems: some subsystems can be encapsulated and their mutual interactions as well as their interactions with the rest of the system are supervised by a controller.