Assessment of disease status and response to therapy in ankylosing spondylitis is a rapidly expanding area of research. The assessment in ankylosing spondylitis international working group has contributed greatly to this development, defining core sets of health domains for use in daily practice and in clinical trials, developing and validating measurement instruments corresponding to these health domains, and developing response and remission criteria for use in clinical trials. This chapter reviews available measures of three major areas of disease impact in ankylosing spondylitis (disease activity, structural damage and functioning), and discusses which measures are relevant for use in clinical practice.