Most of the cost of a product is determined by its design. Among current design theories and methods, there lacks a formal design method to support the design of a product that can be used as two or more products, an example of which is a sofa bed. Design for Versatility (DFV), as a design theory and methodology, will be developed to help designers to design a product that can perform functions of two or more products. As decreasing waste and surgery time is of high importance in medical fields, DFV will be tested and applied in the design of medical devices that have multiple functions and can be used in multiple times and on people of different physical build. Two case studies in medical devices are chosen to be designed using DFV. Prototypes of new designs will be produced to investigate the efficiency of DFV. Finally, a software tool will be developed to help designers apply DFV to their designs.