In this chapter, we describe what quantitative polymerase chain reaction (qPCR) is as well as a brief history of its development. We then focus on how it is being used in precision medicine and as a companion diagnostic. We utilize examples in oncology and infectious disease to exemplify how qPCR has been successfully utilized in the past. We provide evidence for the robustness of qPCR as a companion diagnostic and for its potential to become a companion diagnostic standard. We also address how qPCR can be used in the future, some of the limitations associated with qPCR, and how some of these limitations can be addressed.