Tolerance design is the process of deriving a description of geometric tolerance specifications for a product from a set of specifications on the desired properties of the product. Existing approaches to tolerance analysis and synthesis entail detailed knowledge of geometry of assemblies and are mostly applicable during advanced stages of design, leading to a, less than optimal design process. During the design process of assemblies, both assembly structure and associated tolerance information evolve continuously and significant gains can be achieved by effectively using this information to influence the design of an assembly. Any pro-active apprbach to the assembly or tolerance analysis in the early design stages will involve decision making with incomplete infomation models. In order to carry out early tolerance synthesis and analysis in the conceptual stages of the product design, we need to devise techniques for representing function behavior-assembly models that will allow analysis and synthesis of tolerances, even with the incomplete data set. A 'function' (what the.system is for) is associated with the transformation of an input physical entity into an output physical entity by the system: The problem or customer's need, initially described by functional requirements on an assembly, and associated constraints,on the functional requirements derives the concept of an assembly. This specification of, functional requirements and constraints define a functional model for the assembly. Many researchers have studied functional representation (function based taxonomy and ontology), function to form mapping, and behavior representation (behayior means how the system/product works)., However, there is no comprehensive function-assembly-behavior (FAB) integrated model In this paper, we discuss the integration of function, assembly, and. behavior representation into a comprehensive information model (FAB models). To do this, we need to develop appropriate assembly models and tolerance models that would enable the designer to incrementally understand the build-up.or propagation of tolerances (i.e., constraints) and optimize the layout, features. or assembly realizations. This will ensure ease of tolerance delivery.