Quantitative Systems Pharmacology (QSP) integrates biological knowledge of disease pathology and drug mechanism of action into a mathematical or computational framework to simulate patient responses to drug treatment. QSP is used to support decision making across all stages of drug development, and has been increasingly used in regulatory submissions since 2013. Due to the heterogeneous nature of disease and treatment response, and because of unknown aspects of the relevant biology, uncertainty quantification is critical to meaningfully interpret QSP results and predictions. In this talk, I will give a brief introduction of the field of QSP, applications of uncertainty quantification in QSP such as sensitivity analysis and virtual populations, and will show an illustrative example applied to a live biotherapeutic product (LBP). In this example, we developed a novel cellular kinetic-pharmacodynamic quantitative systems pharmacology model of an LBP targeting the gut microbiome. The model is calibrated and validated to published data from healthy volunteers, then used to simulate the impact of treatment dose, frequency, and duration as well as antibiotic pretreatment on a key therapeutic metabolite.