The understanding of inclusion in education has transcended the assumption that inclusion is about students with special needs. It concerns the inclusion of all children. With systems theory as a framework, the article argues that in order to handle inclusion as a phenomenon that concerns all children, we need an operational definition of inclusion differentiated according to three dimensions. The first dimension covers different levels of inclusion. The second dimension concerns different types of social communities in and out of school, from which a child may be included or excluded. The class is one such type of social community, but equally important is membership in the self-organised community of children in the school-yard, the bilateral relationships with other children and/or teachers, etc. The third dimension concerns different degrees of being included in and/or excluded from the different communities. The point is that a child is not either completely included or excluded, but that he/she is included in or excluded from the different communities in different degrees. A comprehensive matrix definition is presented combining the three dimensions, which matches the present understanding of inclusion in education.