XASM- An Extensible, Component-Based Abstract State Machines Language. The Abstract State Machine (ASM) [15] approach has already proven to be suitable for large-scale specifications of realistic systems [17,8,20,33]. Due to the fact that the ASM approach defines a notion of executing specifications, it provides a perfect basis for a language, which can be used as a specification language as well as a high-level programming language. However, in order to upgrade to a realistic programming language, such a language must - besides other features - add a modularization concept to the core ASM constructs in order to provide the possibility to structure large-scale ASM-formalizations and to flexibly define reusable specification units. In this paper, the language XASM, which stands for Extensible ASM, is presented. XASM realizes a component-based modularization concept based on the notion of external functions as defined in ASMs. This paper also briefly describes the support environment of XASM consisting of the XASM-compiler translating XASM programs to C source code, and the graphical debugging and animation tool.