This book provides the current state of knowledge of basic mechanisms of adverse drug reactions (ADRs). The main focus is on idiosyncratic drug reactions because they are the most difficult to deal with. It starts with a general description of the major targets for ADRs followed by a description of what are presently believed to be mediators and biochemical pathways involved in idiosyncratic drug reactions. There is also a description of several examples of ADRs that serve to illustrate specific aspects of ADR mechanisms. Eventually the book shows that ultimately better methods are needed to predict which drug candidates are likely to cause ADRs and which patients are at increased risk. But at present research seems to be far from this goal.