Telecommunication systems are characterised by a common feature, the complexity, which is basically derived from that of the problem domain. This is manifested by the large number of the provided services. Object oriented paradigm offers an attractive way for handling complexity and when it is combined with other emerging technologies it can provide the change needed in the development of complex telecommunication systems. However, although the value of using object oriented techniques for improving the quality and the reliability of complex systems has been recognised, object oriented development is not a common practice in today’s telecommunication industry. Several object oriented methodologies have been examined and evaluated by the telecommunication industry, in order to identify the strongest of them in this specific problem domain. Coad and Yourdon’s methodology has been recognised as one of the strongest candidates. However, it lacks certain features for handling special requirements of this problem domain. In this paper, a number of extensions to this methodology are presented in order to fulfil these requirements. Special attention has been given to task and architectural modelling.