This book constitutes the thoroughly refereed extended postproceedings of the Second International Mozart/OZ Conference, MOZ 2004, held in Charleroi, Belgium in October 2004. Besides the 23 papers taken from the workshop, 2 invited papers were especially written for presentation in this book. The papers are organized in topical sections on language-based computer security, computer science education, software engineering, human-computer interfaces and the Web, distributed programming, grammars and natural language, constraint programming, and constraint applications.