Clinical measurement is fundamental to patient diagnostics, and good measurement equals good science. This chapter outlines the factors pertinent to good clinical measurement within a healthcare environment. The concept that a medical device can be precise but not accurate is explained, and that calibration against a traceable standard before use is crucial to accurate patient data interpretation. The concept that measurement error is a combination of human, systematic, and random errors summated is introduced. Measurement uncertainty is introduced, and the process of using Type I and Type II errors in hypothesis testing is described.