The conceptual basis to summarize and predict the in vivo behavior and the related mathematical descriptions and models of creatine and creatinine are currently based on the principles and the methodologies of pharmacokinetics and pharmacodynamics. Most in vivo kinetic analyses and models of creatine and creatinine are assumed linear. The presence of nonlinear in vivo kinetics for creatine are assessed by the author due to the affinity and preferential uptake of the molecule in the muscle. This book is an inquiry into the application of these principles and methodologies for the endogenous compounds creatine and creatinine, and provides new pathways for studies and bench research.