Systems engineering is a mandatory approach in some industries, and is gaining wider acceptance for complex projects in general. However, under the imperative of delivering these projects on time and within budget, the focus has been mainly on the management aspects, with less attention to improving the core engineering activity - design. This book addresses the application of the system concept to design in several ways: by developing a deeper understanding of the system concept, by defining design and its characteristics within the process of engineering, and by applying the system concept to the early stage of design, where it has the greatest impact. A central theme of the book is that the purpose of engineering is to be useful in meeting the needs of society, and that therefore the ultimate measure of the benefit of applying the system concept should be the extent to which it advances the achievement of that purpose. Consequently, any consistent, top-down development of the functionality required of a solution to the problem of meeting a defined need must proceed from such a measure, and it is agued that a generalised form of Return on Investment is an appropriate measure. A theoretical framework for the development of functionality based on this measure and utilising the system concept is presented, together with some examples and practical guidelines.