Renal physiology is at the heart of practice of medicine. Concepts such as fluids, electrolytes and acid/base disorders are central to medical disciplines in almost all fields of medicine. There are very few physicians who do not need to be well versed in these concepts. On the other hand, these concepts are some of the most poorly understood concepts in medicine. As an example, there is often confusion that has led to hyponatremia, and physicians are often in doubt as to what fluids are required for a patient with hypernatremia. This book will clarify the logic behind these central concepts and hopefully lead to less doubt in the management of patients with these problems. The book is not intended to be a comprehensive discussion for all aspects of renal physiology; rather, it is intended to clarify the understanding of few core concept of renal physiology as it relates to patient care. As each patient with electrolyte or renal disorder presents in their own unique way, we find it useful to understand the basics behind those core concepts to be able to explain why the patient does not completely fit the textbook case. The purpose of this book is not to serve as a textbook on renal disorders. As such, not all topics in nephrology are covered, but only the ones where we find it beneficial for the physician to better understand those aspects of renal physiology. The book is intended for all physicians; clearly, medical students in their clinical years would benefit from it and especially renal fellows and nephrologists would find it useful. It is often the case that physicians act as the developers of renal physiology and equations, but not much time is spent on understanding how those equations and concepts came about. This book is intended to shine light on these important concepts. Having a true understanding of these concepts would enable one to treat patients who often don’t present as a textbook case. In general, this textbook would be helpful for all physicians. However, the group of physicians who would benefit most from it would be those who encounter patients with electrolyte disorders. First and foremost, nephrologists are included in that list, especially nephrology fellows who are just starting to develop a deeper understanding of serum electrolytes. The next group of physicians who would benefit would be intensivists, internists, family practitioners and emergency room physicians who often act as the first line of responders for these patients. Our book is unique among books on renal physiology in that it is not a comprehensive discussion of renal physiology, but it gives the physician reader some helpful hints in understanding key concepts of renal physiology. As such we believe it would be especially helpful in the management of patients with complicated electrolytes or renal disorder.