Software architecture research focuses on models of software architectures as specified in architecture description languages (ADLs). As research progresses in specific areas of software architectures, more and more architectural information is created. Ideally, this information can be stored in the model. An extensible modeling language is crucial to experimenting with and building tools for novel modeling constructs that arise from evolving research. Traditional ADLs typically support a small set of modeling constructs very well, but adapt to others poorly. XML provides an ideal platform upon which to develop an extensible modeling language for software architectures. Previous XML-based ADLs successfully leveraged XML's large base of off-the-shelf tool support, but did not take advantage of its extensibility. To give software architecture researchers more freedom to explore new possibilities and modeling techniques while maximizing reuse of tools and modeling constructs, we have developed xADL 2.0, a highly extensible XML-based ADL. xADL 2.0 supports run-time and design time modeling, architecture configuration management and model-based system instantiation. Additionally, xADL 2.0 has a set of extensible infrastructure tools that support the creation, manipulation, and sharing of xADL 2.0 documents.