Conceptual Design methods place some intermediate solution steps between problem definition and final technical solution. Functional Decomposition is such a method which aims at finding solution concepts in terms of functions first and, after that, technical realisations for the individual functions. The role of mathematical models in conceptual design is to provide some coarse quantitative assessment of certain solution concepts, and furthermore, to clarify the relationship between the Functional Requirements and the Design Parameters. Later on, in the more detailed design steps refined mathematical models are used. What is their relation to the early models used in conceptual design? The concept of perturbation analysis seems to provide a framework for defining and understanding these relationships.