: This paper describes a method aiming to support the design of interactive-safety critical systems. The method proposes an original integration of approaches usually considered separately, such as task modelling and distributed cognition. The basic idea is that analysing task performance requires a clear understanding of the information needed to accomplish the task and how to derive such information from both internal cognitive representations and external representations provided by various types of artefacts. We also report on a first application of the method to a case study in the Air Traffic Control (ATC) domain. Key words: Human-Computer Interaction, Methodologies, Design, Task Models.