The basic concept of this work is that functional modelling techniques are applicable to and of practical use in, producing a qualitative model of conceptual engineering design. A qualitative function based model of conceptual design has been developed and a computer based implementation has been built and tested. The rationale behind the modelling scheme and the computer implementation are described in detail. In addition to a review of existing models of design the research provides a significant new capability in four main areas: • An ability to generate new concepts with a controlled degree of similarity to existing designs. • A new function based model of engineering conceptual design. • The COncept Design ASsistant (CODAS) system, a computer based implementation of the function based model, has been developed and tested. • A new symbolic representation language. CODAS is a hybrid case-based and function-based modelling system, implemented in the domain of mechanical device design, which demonstrates the practical application of this new model. The CODAS system aims to provide a design support tool which can invent both routine and novel devices based on experience gained from past successful design solutions. Fast and efficient data handling is achieved by utilizing Case Based Reasoning (CBR) technology to store and retrieve past design solutions which are defined in terms of a symbolic representation language. The underlying design model is function based and employs a technique of divergent function to form mapping to produce physical embodiments of the proposed functional solutions.