Modular design can be achieved by taking the variations of the surrounding components as conceptual noise and designing the part to be conceptually robust. Design-in-modularity, which has demonstrated its success in electrical/electronic industry, receives less attention in the other engineering disciplines. Previous work focuses on descriptive discussions as well as coding and classifying designs based on product features; none has outlined a design method for modular design. This paper articulates a procedure for design-in-modularity based on the idea of conceptual robustness. We claim that, by properly defining the conceptual noise, the conceptually robust design is the modular design. Issues on how to generate the minimum number of modular designs for a given set of systems are included. An algorithmic procedure is outlined for the practitioners to follow. This paper also discusses future research issues in this new research area of interest. The entire concept is illustrated by an example.