This book is a collection of updated and enhanced versions of the authors articles published in English over the past 30 years. It covers a wide range of areas and deals with many practical problems of international law which, despite their diplomatic importance, have been neglected by academics. For example, the book explores the applicability of negotiorum gestio to the planetary climate crisis, international legal problems between sovereign wealth fund holders and recipients, and the possibility of using the frozen assets of the Russian Central Bank. Through joint research and discussions with diplomats and government officials, the author came to realise that what really matters in diplomacy is different from what academics are interested in. On the basis of this conviction, he has included here articles which he considers useful not only for the academic study of international law but also for diplomatic practice. This book covers broad areas of international law and also touches on Japanese ideas of international law.