User design is a particular form of product customization that allows the customer to specify the properties of a product. User design has emerged as a mechanism to build brand loyalty, to fit products to the heterogeneous needs of a market, and to differentiate the offerings of a manufacturer. However, many consumers face daunting challenges in designing a product that fits their personal needs. This makes it essential for producers of customized goods and services to create user interfaces that are effective in supporting consumers in the user design process. This article defines the fundamental information- processing problem associated with user design of customized products and articulates five principles of user design. It then outlines actions that can be taken to improve user design systems.