Reverse transcriptase (RT) is the key enzyme required for conversion of RNA to DNA. Cloning of Moloney murine leukemia virus (MMLV) RT has enable engineering an RT that lacks endogenous RNase H activity. RT catalyzes cDNA synthesis more efficiently in the absence of RNase H. We describe here a number of properties of MMLV RT and RNase H-minus MMLV RT not summarized in a single location elsewhere, providing a basis for best use of these enzymes in cDNA synthesis. In addition, general guidelines and detailed protocols are provided for use of MMLV RTs in one tube double-stranded cDNA synthesis, in [32P]cDNA synthesis, and in RT-PCR and long RT-PCR.