Quantitative systems pharmacology (QSP) is a modeling approach employed in drug research and development that combines mechanistic representations of biological processes with drug pharmacology to deepen biological understanding and predict the responses to novel drugs or protocols. QSP has evolved from and is related to other modeling approaches, but has a number of unique attributes and applications. Here, we clarify the definition of QSP and its key features, trace its evolution, briefly compare it to other approaches, and explain why and how it can be used to reduce risk and improve efficiency in drug research and development.