Tolerance stackups rank among the more important tools available to a design team for gaining insight into a product design. Careful allocation of tolerances between the features in a product help balance the often conflicting requirements of design and manufacturing. Prototypes of new products are expected to assemble and function with little difficulty. When designing a new product that is part of a large product family, tolerance stackups are even more important. The sub-assemblies that make up a product family must be interchangeable and the interactions between sub-assemblies must be well understood. This paper will discuss various aspects of stackups in the design process along with a number of real-life factors that complicate the stackups.