Formal methods have been established as the rigorous engineering methodology for the system development. Applying formal methods to a large and complex system development often requires the modelling of different aspects of such a system. For instance, complex systems (such as integrated avionics systems, engine control software) can involve functional and timing requirements that must be eventually implemented as executing code on a communicating distributed topology. This book contains the papers presented at the First International Workshop on Integrated Formal Methods, held at the University of York in June 1999. The conference provided a forum for the discussion of theoretical aspects of combing behavioural and state-based formalisms and practical solutions to the industrial problems of this approach.