The Designers' Workbench is a system, developed by previous research to support designers in large organizations, such as Rolls-Royce, to ensure that the design is consistent with the specification for the particular design, as well as with the company's design rule book(s). The evolving design is described against a jet engine ontology. Design rules are expressed as constraints over the domain ontology. To capture the constraint information, a domain expert (design engineer) has to work with the knowledge engineer to identify the constraints, and it is then the task of the knowledge engineer to encode these into the Workbench's knowledge base. This is an error-prone and time-consuming task. It is highly desirable to relieve the knowledge engineer of this task, and so this thesis proposes a novel approach to facilitate domain experts in capturing and maintaining constraints. The approach has been embodied by developing a system, ConEditor that facilitates domain experts in combining selected entities from the domain ontology with keywords and operators of a constraint language to form a constraint expression.