Quantitative systems pharmacology (QSP) is an interdisciplinary approach that integrates mathematical modeling, biological knowledge, and experimental data to gain insights into the complex dynamics of drug action and disease progression on the human physiological system. Different types of data from multiple and often disparate sources are used throughout all phases of QSP model development. Herein, we explore the various types of data used to inform the development and application of QSP models, their sources, and how the quality of data impacts the interpretation of model-derived results.