The modeling of quantitative structure-activity relationships (QSAR) is a very useful approach in establishing a direct relationship between the physico-chemical properties and the biological activities of the studied species. They, therefore, act as trustworthy statistical tools in predicting the biological property of new species. The structural alteration, which causes the variation in biological properties, is the main driving force in building QSAR. In this chapter, we have reviewed the different approaches in constructing QSAR and their successful application in predicting biological activity and toxicity of different class of molecules. Their scope of applicability in medicinal chemistry toward drug design and the limitations therein have also been highlighted. Special attention has been drawn to represent the effective modeling of QSAR based on different global and local reactivity descriptors of conceptual density functional theory.