Object-oriented design methods are commonplace in computing systems development, but are often dismissed as little more than 'boxes and arrows'. If systems developers are to gain the full advantage from such methods, they should be able to achieve designs that are not merely the subject of heated argument, but can be improved by careful, rigorous and machine-supported analysis. Validated Designs for Object-oriented Systems describes an object-oriented design approach that combines the benefits of abstract modelling with the analytic power of formal methods, to give designs that can be rigorously validated and assured with automated support. Aimed at software architects, designers and developers as well as computer scientists, no prior knowledge of formal methods is assumed. The elements of functional modelling are introduced using numerous examples and exercises, industrial case studies and experience reports. Industry-strength tools support the text, and further materials are available on the accompanying website.