The world is changing. The era of globalization – where the states have been eager to cooperate towards common aims – might seem to be disappearing. Russia expresses a contempt for international law, China makes claim to power, while the United States, at least temporarily, shows disregard to international institutions they themselves created (in particular, the WTO and the WHO). The growing phenomenon of nationalistic and authoritarian leaders may seem to hamper the development of community interests. Some states may be more inclined to resort to reciprocal behaviour as certain events indicate (e.g., the exchange of prisoners-of-war in the Russia-Ukraine conflict and an exchange of economic sanctions – tit-for-tat or tat-for-tat). The field of international law appears to be returning to first principles in the light of multiple world crises. Thus, reciprocity – as a classic concept of international law – may “come back from the shadows,” be reinvented and reinforced to form one of the basic pillars in the relations among states. Given the above, the purpose of the book is to rediscover and identify the position and role of reciprocity in contemporary international law.