Quantitative Structure Activity Relationships (QSAR) relates the biological activity of compounds to their physicochemical properties using mathematical models. QSAR equations express how properties like hydrophobicity, electronic effects, and steric hindrance of molecules and substituents influence biological response. Hydrophobicity is often quantified by log P values, while electronic and steric effects use constants like Hammett's s, Taft's Es, and molar refractivity. Successful QSAR equations incorporate multiple physicochemical terms to predict activity for new structures within the same class.