There is an increasing awareness of the importance and the benefits to be attained from consulting the end-users during system and product development. Although the rationale of utilizing the expertise of end-users in the system development life cycle appears to provide an apparently straightforward, even 'ideal' approach, there are many difficulties associated with eliciting the required knowledge from experts, both general and specific to every user group. Furthermore, many developers do not know how to involve users, or if they do, they do not utilize them to best effect. In the avionics sphere, the wealth of knowledge possessed by line pilots and flight engineers represents a vital information resource for the design of future flight-deck systems. As a specific example, this paper overviews some of the considerations which arose from working with these end-users in the early stages of the development of a warning and diagnostic system for civil aircraft. The end-goal of this particular phase of the work was the generation of guidelines for the design of the interface for the software engineers to use when building the prototype, and the methodological approach taken to achieve this is reported here.