We applied the cognitive systems engineering (CSE)/ecological interface design (EID) approach to the work domain of flexible manufacturing systems (FMS). Work domain analyses (WDA) and activity analyses were conducted. Previous concerns regarding the suitability of CSE for FMS are addressed. Principles of EID (i.e., direct perception, direct manipulation, and visual momentum) were applied in designing an ecological interface for a flexible manufacturing system (EcoFlex). The critical links between the products of a WDA and the content of an ecological interface (a key innovation of EID) are made explicit. The evaluations of this interface were very positive across a wide variety of traditional and novel dependent variables (please see the companion manuscript). The overall success of this research program clearly demonstrates that the CSE/EID approach is capable of being applied to FMS. Contributions of this manuscript include both theoretical insights (defining properties of the FMS work domain, concrete examples of the principles of EID, critical links between CSE and EID, resolution of concerns about the overall framework) and practical applications (specific design solutions for FMS and similar work domains, and successful examples of the CSE/EID approach which can be studied and leveraged by students).