Publisher Summary This chapter elaborates studies that focus on reverse transcription and its inhibitors. Reverse transcription is the name given to the process of complementary transcription from a polyribonucleotide strand into a polydeoxynucleotide. Analysis of the assay conditions in vitro generally employed for reverse transcriptase determination reveals a variety of standard substrates and of interacting substances, which influence observed rates of catalysis or measured inhibitor/activator effects. Only a few of the drugs, reported as reverse transcription inhibitors, have so far been found to block production of new progeny virions of oncorna virus. These include streptovaricins, cactinomycins, anthracyclines, and rifamycins. Observations of progeny production blockade strengthen the hypothesis that RNA→DNA synthesis is a “sine qua non” of oncorna virus reproduction, but do not assure it because these agents might affect some other polynucleotide transcriptions to some degree as well. Chemicals blocking the reverse transcription process serve not only as probes into its significance, but also as drugs with the power to prevent selected types of in vivo virus-induced cancer, virus replication, and antibody production in animal model studies. More selective and specific drugs, which act as reverse transcriptase inhibitors, need to be developed and a variety of structure classes of compounds so far studied have provided important leads.