Serious disturbances of fluid and electrolyte balance are frequently encountered in acutely ill patients; somewhat less often in the chronically sick. There seems to be a trend for such cases to increase, due probably to an increase in major surgical procedures on older patients whose renal function is less than adequate. There are already many publications dealing with the physiology of the homeo­ stasis of fluid and electrolytes, and others dealing with the clinical aspects of the subject. It is often assumed that a knowledge of the basic principles of physiology will enable the doctor to prescribe suitable intravenous therapy. In practice this is often found not to be so and the evidence for this is the frequency of calls for help with electrolyte problems from well-qualified and experienced doctors who are undoubtedly equipped with adequate or even excellent knowledge of the basic It is not an unusual observation that knowledge of theory and principles involved. principles does not necessarily lead to successful practice in this or any other art or craft. Most doctors already possess knowledge of the physiology of the internal envi­ ronment, but some are aware of being unable to deal effectively with clinical problems related to fluid and electrolyte disturbances and seek guidance to translate theoretical knowledge into practice.