When designing a study, it is important to select appropriate instruments to measure health outcomes. An investigator must have a comprehensive understanding of the disease and its effects on patient health to inform instrument selection. We provide a brief introduction of different ways that health is defined and the properties of a good measurement tool (reliability, validity, sensitivity to change). We describe patient-reported outcomes (PROs) and methods to improve their interpretability, and we include strategies to reduce bias in health measurement and considerations that may improve the feasibility of measuring outcomes in a research study.