The practical aspects of population pharmacokinetics / pharmacodynamics (PK/PD) are summarized. Recently, the progress and application of Model-Based Drug Development (MBDD) in pharmaceutical industries is remarkable and the acceptance of model-based results are getting wider in regulatories of US, EU and Japan. In MBDD, PKPD modeling based on nonlinear mixed effects model (NLMEM) plays an important role, and it provides the quantitative results which help the decisionmaking in projects. The

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... erstanding of its methodology is quite important for pharmacometricians and biostatisticians. In this article, objective, design and modeling methodology (scheme, diagnostics and validation) are briefly described. The research on methodology in NLMEM is still growing, and pharmacometricians should follow its rapid evolution.