Testing people to see if they are currently infected or previously infected with SARS-CoV-2, the virus that causes COVID-19, is a key component of medical management, public health monitoring and research. Diagnosing people as having active infections is a fundamental part of any test and contact tracing system. Improving the speed and accuracy of tests that detect current infections is a research priority and the focus of recent UK Government investment and policy decisions. Antibody tests are also an important tool to understand how many people in the population have been infected and how their immune system responded. This article explains how the accuracy of tests is determined, how different test characteristics suit different purposes, and the strengths and weaknesses of using them in different contexts.