The aim of the work is to provide better computer aided support for design and manufacturing in order to enhance process integration, reduce product development cycles, increase quality and lower costs, thereby raising commercial competitiveness. Feature Modelling is accepted as an aid to process integration and simultaneous engineering strategies. However, there are a number of problems restricting the impact of feature modelling technology in Computer Aided Design and Manufacturing. There is a considerable legacy of purely volume based models. Libraries of predefined features are too restrictive. There are no standards for the exchange of user defined feature models. Engineering drawings are provided in 2D representations separate from the feature model. Tolerances and dimensions are generally applied to engineering drawings, hence are not homogeneous with the three-dimensional (3D) shape model which precludes the development of variational models and enhanced design and manufacturing analysis. User defined feature functionally has been neglected. The following objectives have been undertaken in this work in order to address the above problems: Development of methods to model user defined features with extrinsically defined form; Implementation of a 3D dimension and tolerance system associated with solid and features models; Establishment of techniques to control a feature's parameters through the use of dimensions; Investigation of structures for the exchange of user defined feature models. Extrinsic form features have been developed that allow the integration of objects from many different sources within the feature model that can be swept to create free form geometry. Any solid or feature model may have associative dimensions and tolerances applied which are visualised as part of the 3D model. The template definition of a feature has been extended to enable dimensions representing a features size parameters to be described; these dimensions can be used to control the model. The dimensions and geometric tolerances have been implemented as features belonging to separate workpieces creating a hyper feature model. Dimensional tolerances have been implemented as attributes of the dimensions. Parallel geometric tolerances have been found to behave as 3D constraints. The feature and accuracy model has been output to manufacturing software applications.