The flexibility of an automated manufacturing system (AMS) is largely determined by the architecture of its supervisory software. The AMS supervisory software should be compatible enough to allow re-configuration, maintenance, and upgrading to be done at a reasonable cost, so as to prolong the life span of the AMS, and to maintain its value and success, rn this connection, the application of software engineering methodologies is to introduce better management of AMS software development, with a view to providing some structure and formality to express, analyse, and produce them. This paper presents an object-oriented methodology to address the task of AMS supervisory software development. The effectiveness of the methodology is examined in an extensive case study of the development of the supervisory software for a flexible manufacturing cell (FMC) that contains a robot and an AGV.