Significant problems exist in understanding and implementing the Mental Capacity Act 2005 (MCA) despite it having been in place in England and Wales for more than 15 years. Although many guidelines exist on the assessment of capacity, it is difficult to apply such guidance in practice, given the complexity of actual situations. We may know the stages of the test for capacity, but do we really understand what they mean and how to assess them? In this, the first of two articles examining the MCA and challenges in its application, we add to existing legal guidance to explore capacity assessment in detail using a clinical scenario, and use case law and case studies to illustrate the subtleties and difficulties that may arise.