There is no legal instrument specifically designing peacekeeping operations. These operations form part of what legal science calls the subsidiary bodies of international organisations and are not expressly provided for in their founding charters — with the exception, in the case of the EU, of the Treaty of Lisbon. Any definition of what PKOs are must be established on an ad hoc basis and based on practice. Without disparaging other international institutions, it is the United Nations, by its universal nature, that can and must harmonise any efforts to achieve such peacekeeping and international security. However, the United Nations cannot assume a monopoly of international peacekeeping and security. For this reason, this book analyses the peacekeeping activities of the regional organisations, such as the EU, OSCE, NATO, or any other. Diverse and specific local experiences have been very useful for peacekeeping. This has been the case with the new functions taken on by PKOs such as protection of the cultural heritage, participation of the World Bank, the new war on terrorism, etc., all of which require new analyses that can be included in a more functional context. Any measures adopted in peacekeeping missions must take gender into account, given its enormous impact and it is therefore essential to examine this issue. These aspects are included in the book. The study of the impact peacekeeping operations have on human rights, on the financial conditions of the people living in the places to which peacekeepers are deployed, on child protection, etc. are others issues covered within this book.