With the increasing differentiation of organisations involved in the pursuit of public health, there is also a growing need for inter-organisational integration. Starting from the concepts of differentiation and integration, this article is attempting a theoretical reconstruction based on published research on inter-organisational integration in public health and related welfare services. Different forms of integration are defined and related to each other in a conceptual framework, which is in itself an integration of different theoretical perspectives. According to this framework, integration in the field of public health requires inter-organisational collaboration across different sectors of the society. Such intersectoral collaboration can be organised mainly in the form of multidisciplinary teams across the boundaries of different organisations and sectors. Such an organisation is fragile and volatile, however, which means that it needs a lot of management support in order to survive.