All previously published papers were reproduced with permission from the publisher.Published and printed by Universitetsservice AB Nanna Swartz väg 4, SE-171 77 Stockholm, Sweden ABSTRACT Participation in everyday life is vital to a child's development and well-being and is a basic human right. One definition of participation is engagement in life areas. The objective of this study is to investigate participation in school activities of children and adolescents with disabilities; the study focus on personal factors and factors in the environment, which are closely related to participation. Data were collected in a large survey and a smaller observational study.In the survey, students, parents, teachers, and special education consultants responded to statements about participation and factors related to participation such as autonomy, interaction, availability, support, and environment.In the observational study, participation was observed during various school activities during an entire school day and after school. Children were interviewed about their school day, friendships, and autonomy.The results revealed that participation is multidimensional, with an emphasis on personal experiences, interactions, and context. The theoretical assumption of the International Classification of Functioning, Disability, and Health (ICF) states that the body, participation, and the environment are related. The research results proved this assumption and support the multidimensionality of the participation construct. As indicated in previous research, children and adolescents with disabilities show a lower degree of participation in school than their peers. Participation seems to be more related to autonomy and interactions with significant others than to disability type and general environment. A closer look at various school activities reveals that children with disabilities primarily have a lower degree of participation in math, practical subjects, and science. Being included and having many friends, who provide emotional support, facilitate participation. While, frequently receiving support from teachers and assistants lowered participation. This indicates that there is a relation between support and participation: providing too much support during class can be detrimental to class participation, whereas a good social support network of other children is vital.In this thesis, participation is measured in two ways: by participation frequency and by participation intensity. By counting the number of activities that children participate in, and how often they participate in these activities, a measure of an individual's average participation is obtained, that is, participation frequency. This measure depends more on internal rather than contextual factors, and it changes more often because of internal rather than contextual changes. Intensity measures of participation refer to the amount of involvement within a specific situation, and are contextually dependent. Involvement change based on the situation and the individual's pr...