The year 2020 has seen a major and sustained outbreak of a novel betacoronavirus (severe acute respiratory syndrome (SARS)-coronavirus (CoV)-2) infection that causes fever, severe respiratory illness and pneumonia, a disease called COVID-19. At the time of writing, the death toll was greater than 120 000 worldwide with more than 2 million documented infections. The genome of the CoV encodes a number of structural proteins that facilitate cellular entry and assembly of virions, of which the spike protein S appears to be critical for cellular entry. The spike protein guides the virus to attach to the host cell. The spike protein contains a receptor-binding domain (RBD), a fusion domain and a transmembrane domain. The RBD of spike protein S binds to Angiotensin Converting Enzyme 2 (ACE2) to initiate cellular entry. The spike protein of SARS-CoV-2 shows more than 90% amino acid similarity to the pangolin and bat CoVs and these also use ACE2 as a receptor. Binding of the spike protein to ACE2 exposes the cleavage sites to cellular proteases. Cleavage of the spike protein by transmembrane protease serine 2 and other cellular proteases initiates fusion and endocytosis. The spike protein contains an addition furin cleavage site that may allow it to be 'preactivated' and highly infectious after replication. The fundamental role of the spike protein in infectivity suggests that it is an important target for vaccine development, blocking therapy with antibodies and diagnostic antigen-based tests. This review briefly outlines the structure and function of the 2019 novel CoV/SARS-CoV-2 spike protein S.