For any sort of computer system, the problems of being sure you have asked for the right thing and then being sure you are implementing the right thing are important and hard problems. For systems with a graphical user interface there are the analogous additional problems of making sure that the interface allows any interaction that is required, and works in a usable way. Design guidelines are used in both the design and evaluation of user interfaces to try and ensure that the systems we build are both usable and conform to specific requirements. This paper discusses practical ways in which we can use formal methods to model guidelines for interface design and then use these as a basis for the formal proof that a specified system has the desired properties described in the guidelines.