In this chapter, methods to isolate RNA and DNA from human leukocytes for the subsequent use in molecular diagnostic tests are described. In addition, protocols for cDNA synthesis are given, both for the use in conventional reverse transcription (RT)-polymerase chain reaction (PCR), and for the use in quantitative RT-PCR reactions. Because sensitive PCR methods are commonly used in molecular diagnostics, measures to avoid contamination are described. Finally, a simple procedure to control the quality of isolated RNA is described.