Pharmacological outcome and a certain side effects of therapeutic drugs generally depend on concentration of the drugs and/or their active metabolites in the body. Physiologically-based pharmacokinetics is quantitative tool to understand the drug concentration in the body. Drug efficacy is sometimes affected by subjective factors and cannot be clearly quantified. Even in such cases, it could be possible to quantitatively understand possible pharmacological events occurred in the patients by understanding pharmacokinetics of the corresponding drug. Here, we have attempted to summarize the basis of physiologically-based pharmacokinetics to understand which factors will determine drug concentration in the body and how to predict/speculate the drug concentration in the body in a quantitative manner. For easier understanding by the readers, we introduce some examples of pharmacokinetic property of several osteoporosis drugs.