Direct measurement of how people are feeling and the extent to which they are functioning in daily activities (generally as patient reported outcomes) is critical to judging the benefit of health interventions in chronic conditions. Selection of an appropriate instrument will reflect a comprehensive understanding of the condition of interest and a thorough knowledge of the expected benefits and harms of the proposed intervention. We provide a brief discussion about different ways that health and health measurement have been defined, including the International Classification of Function, Disability and Health (ICF), Health Related Quality of Life (HRQOL) and cost-to-benefit analyses. We outline important properties (reliability, validity, and responsiveness) that a measurement instrument must demonstrate depending on the purpose of measurement, and provide insight as to how to interpret the results of studies that report patient reported outcomes.