This chapter reviews key theories on achieving regional cooperation and integration. Neofunctionalism emphasizes that collaboration in non-political areas, such as trade or culture, fosters political integration when cooperation in one area spills over into others. The New Regionalism broadens this perspective, focusing on the role of non-state actors and regional identity, as well as the positive effects of regionalism on governance and market dynamics. Comparative Regionalism critiques the Eurocentric view of regionalism, recognizing the unique context of each region and the diversity of integration perspectives. It highlights the significance of both state and non-state actors. Game Theory highlights the role of imperfect information, the primarily conflictual character of international relations, and the need for a conscious effort to overcome it. It is used to analyze trade negotiations and security dilemmas. The Principled Negotiations theory advocates shared interest-based approaches prioritizing common benefits and constructive dialogue over rigid positions. Diplomatic endeavours support regional cooperation in the establishment and maintenance of regional institutions, as well as in the provision of a regional diplomatic service. These theories underscore the complex dynamics of regional cooperation. The categorization of regions and regional integration projects in this book helps handle this complexity.