The current legal test for capacity under the common law is a functional test. It determines whether a person can make a particular decision at a particular time; it focuses on retained abilities rather than on presumed deficits - in order to promote patient autonomy in decision-making. International evidence suggests however that the test is not well understood and consequently not being correctly applied in practice by a range of medical professionals. Donnelly has argued that gaining a better understanding of how capacity assessments are actually employed is essential to improving the quality of assessments. To date, there is little data on how doctors in particular make their assessments in practice. Using a socio-legal methodology, this quantitative study aims to assess understanding of capacity among a group of general practitioners/family medicine doctors in Ireland, and to determine how they assess capacity in clinical practice. Our study found that notwithstanding a lack of confidence in their ability to assess decision-making capacity, that respondents in fact demonstrated a high level of understanding and of ability to correctly apply the functional test in practice.