A prognostic factor is any variable associated with a subsequent outcome such as death or disability among people with a disease or health condition. Prognostic factors range from simple measures, such as age, gender, temperature, or pulse rate, to test results such as X-rays or psychological scores, whilst novel biomarkers and genetic information are increasingly studied. Different values of a prognostic factor are associated with a different prognosis and can be used to stratify overall prognosis estimates. This chapter details the potential use of prognostic factors (including disease definition, identifying new intervention targets, and providing building blocks for prognostic models); the design of exploratory and validation cohort studies to identify prognostic factors; the importance of examining the prognostic value of a new factor over and above existing factors; consideration of time-dependent prognostic effects; and the use of the REMARK reporting guideline.