Chloroquine and hydroxychloroquine are 4-aminoquinoline derivatives used for treatment of malaria for a very long period and drugs of choice for the present time as well. Although they are on the World Health Organization's List of Essential Medicines for many years, their mode of action is a still a matter of debate. Various theories have been proposed and the most accepted is that they interact with the parasite's ability to digest haemoglobin. Chloroquine and hydroxychloroquine prevent hemozoin crystallization in digestive vacuoles, bind to heme and form a highly toxic complex, which disrupts membrane function. Chloroquine and hydroxychloroquine are also disease-modifying antirheumatic drugs, introduced empirically for the treatment of several autoimmune diseases: rheumatoid arthritis, systemic lupus erythematosus, porphyria cutanea tarda, primary Sjogren syndrome, Lyme arthritis and antiphospholipid syndrome. With their immunomodulatory capacity they prevent disease flares and promote long-term survival. In addition, both drugs exert in vitro antiviral effects against a wide range of RNA-viruses. During COVID-19 pandemia, they were used in clinical trials and emergency use programs for the experimental treatment of this viral disease. Finally, numerous studies have identified antitumor activities of chloroquine, hydroxychloroquine and other antimalarial drugs.