Usability testing is widely used in the commercial world during the process of developing new products, especially software and websites. However, it appears to be rarely used in the development of e-learning in medical education. The focus of usability testing is the user of the particular product and it informs product development by using a systematic process to identify usability problems at an early stage during product development so that these problems can be rectified. Usability testing of e-learning considers the characteristics of the learner, the technological aspects, the interaction and instructional design and finally the context. Testing under the conditions that the e-learning intervention will typically be used is the preferred method but more extreme situations can provide useful information. Product development should be iterative and rapid cycles of testing and refinement are essential to produce an effective e-learning intervention.