There are two types of tests available for COVID-19 that can detect whether a person had it in the past (serology testing,which tests for antibodies against SARS-CoV-2, the virus that causes COVID-19), or whether they have it in the present (polymerase chain reaction (PCR) testing, which tests for active infection). This document is designed to explain the differences between PCR and serology testing, and when one test might be used over another.