The ONS Longitudinal Study (ONS-LS) is a rich source of data that can be used for several kinds of study. Users need to select parts of the ONS-LS appropriate to their study design. This document will help you to work out what selection you need for your study. It looks at three examples of studies using the ONS-LS that illustrate the different ways in which a study population can be constrained by the criteria for inclusion of the study subjects.