The COVID-19 pandemic has brought diagnostics to the attention of the health care professionals and the public.  Diagnostics play a critical role in patient management and disease control but there is a limited understanding of what diagnostics are and how they can be used to improve patient care and enable the implementation of public health measures to control the pandemic.  The diagnostic primer aims to provide a basic understanding of what diagnostics for infectious diseases are, how they work and how they can be used for patient management and disease control.