This chapter focuses on the legislation for England and Wales, but many of the questions and issues raised are familiar to clinicians working within the Scottish legislation, and in other jurisdictions. The Mental Capacity Act 2005 includes a set of guiding principles and definitions of capacity; these are listed in the chapter. There is a two-stage test for the assessment of capacity. First, it must be demonstrated that there is an impairment in the functioning of the mind. Second, the assessment of capacity must address decision making process. There are some standardized assessments of capacity, which may be of assistance in some circumstances. However, the emphasis on the particular decision at a particular time means that individualized assessments is often required. Where an individual is found to lack capacity to make a particular decision, the Mental Capacity Act 2005 provides the decision maker with a legal framework to follow.