This book presents a comprehensive legal and constitutional study of emergency powers from a comparative common law perspective. It is one of very few comparative studies on three jurisdictions and arguably the first one to explore in detail various emergency powers, statutory and common law, constitutional and statutory law, martial law and military acting-in-aid of civil authority, wartime and peacetime invocations, and several related and vital themes like judicial review of emergency powers (existence, scope and degree). The three jurisdictions compared here are: the pure implied common law model (employed by the UK), implied constitutional model (employed by the USA) and the explicit constitutional model (employed by India). The book’s content has important implications, as these three jurisdictions collectively cover the largest population within the common law world, and also provide maximum representative diversity. The book covers the various positions on external emergencies as opposed to internal emergencies, economic/financial emergencies, and emergent inroads being made into state autonomy by the central or federal governments, through use of powers like Article 356 of the Indian Constitution.