This paper presents the latest developments of the Extensible Agent Behavior Specification Language (XABSL), a modular and scalable tool for engineering complex multiagent behavior. It is based on hierarchical finite state machines. By the new extensions the development of cooperative multiagent behavior is supported through language elements which allow to conveniently specify how the state machines of multiple agents interact. Basic properties of XABSL are illustrated in direct comparison with Petri Net Plans and the COLBERT language using examples of basic robot behavior. More complex examples from robot soccer are used to illustrate the new extensions of XABSL. The complete system is available online on the XABSL website (http://www.xabsl.de).