This paper outlines a function-based method for addressing design parameter uncertainty during the conceptual design phase of an engineering system. The method is given the name FUNdesign and represents a set of tools for obtaining and storing sensitivity information of functions from previous designs as well as tools for applying this information to designing new systems. To store the sensitivity information, functional models created using the Functional Basis are first created for previous designs. Performance models for each function in the model are then identified and a sensitivity analysis performed on each model with respect to each design parameter. This sensitivity information and its associated performance models are then stored according to functionality in an engineering design repository. The information stored in the repository is then used to aide the allocation of design and modeling resources during the design of a system with similar functionality. The specific focus areas of this paper are the sensitivity parameters and methods required to store the sensitivity information.