Polish Underground State was an unique phenomenon during World War II. It was established on Polish teritories occupied by Germans and Soviets in 1939. Within almost 6 years Poles created not only undeground army fighting with enemies. They also rebuild in conspiracy public administration in all areas of state's activity, developed secret political parties and their secret parliament, they established also system of underground courts. This secret state was accepted by vast majority of Polish population which stayed loyal to their state. This book presenting this phenomenon showing its legal foundations and continuity with prewar constitutional system.