Population pharmacokinetics or pharmacodynamics is a science researching the variability in drug concentration or effect in a population of patients. The application of population PK/PD methods into drug development and regulation has been expanded for decades. The first step for typical population approach is to develop a population PK/PD model using sparsely sampled data collected from clinical trials. Mixed effect method has been employed to analyze such sparse population data. Mean population parameters and their distribution in relation with patient characteristics(e.g., age, weight, organ function) are given by the population model. Clinical trial simulation is the next step applying the information of population parameters. Researchers can see what the results of various clinical trial scenarios will be like in advance at this simulation step. In this review, three examples of population PK/PD modeling applied to drug development are introduced; 1) Designing Phase II clinical trial using simulated effects from single dose Phase I trial, 2) Approval of a new dosage regimen without clinical trial, 3) Search for quantitative relationship between a biomarker and a surrogate endpoint of diabetes.