This book presents the modern theory of necessity and the current distinctions between necessity and self-defense, dwelling defense and duress. Most legal systems have accepted necessity as general defense. However, justifications for this defense and the distinctions between necessity and tangential in rem general defenses have changed significantly in modern criminal law. In addition, the book presents the function of transformation-of-fault (formerly actio libera in causa) in its modern form. Focusing on the Anglo-American and European-Continental legal systems, the book is divided into four chapters: The Modern Defense of Necessity, Integrating Necessity with the Principle of Fault in Modern Criminal Law, Tangential In Rem Defenses in Criminal Law and their Implications for Necessity, The Function of Transformation of Fault.