Outcome measures for the assessment of patients with ankylosing spondylitis (AS) have been the subject of considerable research in the last decade, largely through the contributions of the ASessments in Ankylosing Spondylitis (ASAS) International Working Group. This review focuses on the measurement of disease activity, physical function and structural damage in AS, both in daily patient care and to measure treatment response in clinical trials. The ASAS Core Sets for assessment in AS are an important tool to guide disease monitoring, and the domains they contain are discussed, along with other possible concepts important to patient care, including imaging and health-related quality of life. In clinical trials, the assessment of disease response to therapy using the ASAS Response Criteria is a valuable means of determining treatment efficacy and allows comparison of response across trials and interventions.