This book offers an in-depth analysis of the problem of mistakes of fact in connection with the law of state responsibility – mistakes which have significant implications for global governance and legal certainty that have yet to be fully mapped out in contemporary international law.  The book begins by defining the underlying problem: the abundance of cases in which states have unintentionally violated international law and the law of state responsibility apparent silence regarding the juridical consequences of such errors. In turn, it addresses three key areas of state responsibility. First, it explores how mistakes of fact can influence the analysis of attribution in international practice. Second, it addresses the implications of mistakes of fact in identifying “breaches� of international law. This includes a range of case studies, such as state responsibility for accidentally shooting down civilian aircraft or for the accidental killing of civilians on the part of state agents. Thirdly, the book delves into how mistakes of fact can preclude responsibility for established breaches of international law, either on their own or within the matrix of existing defenses. This book is primarily intended for researchers focusing on the international law of state responsibility. Its aim is to spark a discussion about how to address important aspects of state practice that existing instruments do not exhaustively codify. As it seeks to provide clarity on existing state practice regarding mistakes of fact, the book will also benefit professionals whose work involves issues of state responsibility in practice.