A lexicon is an essential part of any natural language processing system. The size, content and format of the lexicon is crucial in determining the power and sophistication of a natural language processing system. However, a lexicon which provides comprehensive, consistent and accurate lexical information and which is in a format facilitating fast retrieval is not easily available. This paper reports on a project which aims at the development of such a lexicon. The resulting lexicon is actually the modified and extended version of the machine tractable version of the Oxford Advanced Learner's Dictionary. The modification and extension concentrate mainly on the aspects of comprehensiveness, consistency, explicitness, accuracy and the dictionary format. The modified and extended version is considered a desirable source of lexical information for any natural language processing system.