Regulatory agencies, professional societies, and patient groups have encouraged the inclusion of patient-reported outcomes (PROs) in clinical trials. There is a growing understanding of and encouragement for the use of PROs to capture tolerability. Traditionally, tolerability has been defined as the absence of serious adverse events. More recent and patient-centric definitions of tolerability have been put forward, focusing on how patients feel and function while on treatment. This chapter will provide a comprehensive overview of how to measure, analyse, and visualise data from patients about how they feel and function while on therapy. We will describe the potential use of overall summary measures and multi-item scales for assessing symptomatic side effects. General statistical and practical considerations for tolerability data, as well as key study design and statistical considerations for early-phase and late-phase trials, will be discussed. Issues that will be covered include dose optimisation in early-phase trials and the specification of tolerability endpoints and hypotheses in late-phase trials. We will also discuss key considerations when measuring tolerability in paediatric trials, including paediatric-specific measures and considerations of when children are able to self-report tolerability. Finally, considerations for communicating tolerability data will be discussed.