Most biological assays measure potency of compounds as an activity coefficient, frequently known as molar concentration of inhibitor at 50% response (IC50) or an agonist concentration at 50% biological activity (EC50). Since biological response responses are non-linear and generally sigmoidal in shape, appropriate curve fitting is important. In addition, the variability in measured IC50/EC50 should be monitored to ensure reliable and robust data analysis of the assay curves to support expensive SAR operations in drug discovery. This chapter addresses in detail significant concepts in curve-fitting techniques and statistical concepts and tools required SAR support.