A fundamental aspect of the Mental Capacity Act 2005 (MCA) is the statutory endorsement of a functional approach to capacity. In principle, this requires a separate assessment of capacity in respect of each decision to be made. Most capacity assessments take place at a non-judicial level, and, outside of day-to-day decisions, the most common assessors are likely to be healthcare professionals. This paper investigates the practical operation of the capacity assessment process at both judicial and non-judicial levels. It asks whether the process can deliver on the MCA's goal of preserving maximum decision-making freedom, while, at the same time, providing an appropriate degree of protection. It argues that assessors who are not legally trained encounter significant difficulties in carrying out the essentially legal task of assessing capacity. It also shows that assessors' values and biases, both professional and personal, are likely to impact on the conclusions reached. Having identified these difficulties, the paper considers their implications for the MCA's approach to capacity. It evaluates the attempts in the MCA itself to address assessors' limitations and argues that these will not have a substantial impact on the way in which capacity assessors operate. It considers other ways in which the quality of assessment could be improved and puts forward specific suggestions as to how greater assessor accountability can be delivered and increased rigour introduced to the capacity assessment process.