Angiotensin-converting enzyme 2 (ACE 2) is the entry receptor for the novel coronavirus SARS-CoV-2, the aetiological agent of COVID-19. At the same time, ACE 2 expression decreases during COVID-19. Two seemingly contradictory relationships between the expression of ACE 2 and COVID-19 have been reported. Increased level of expression of ACE 2 may be a risk factor for the development of COVID-19 infection, while reduced ACE 2 expression during COVID-19 leads to acute respiratory distress syndrome. This article provides a comprehensive overview of available scientific knowledge about the role of ACE 2 in the pathogenesis of COVID-19, which is available up to current day. Also, it discusses unknown factors that we will have to reveal in order to understand the whole role of ACE 2 in the pathogenesis of COVID-19.