In practice, object-oriented design models have been less useful throughout the lifetime of software systems than they should be Design models are often large and monolithic, and the structure of designs is generally quite different from that of requirements. As a result, developers tend to discard the design, especially as the system evolves, since it is too difficult to keep its relationship to requirements and code accurate, especially when both are changing. This thesis identifies a number of key, well-defined problems with current object-oriented design methods and proposes new techniques to solve them. The new techniques present a different approach to designing systems, based on flexible decomposition and composition. The existing decomposition mechanisms of object-oriented designs (based on class, object, interface and method) are extended to include decomposing designs in a manner directly aligning design with requirements specifications. Composition mechanisms for designs are extended to support the additional decomposition mechanisms. The approach closely aligns designs with both requirements specifications and with code. It is illustrated how this approach permits the benefits of designs to be maintained throughout a system’ s lifetime.