The aim of this thesis was to describe and analyse principals´ views of collaboration forms, staff development and leadership, as critical internal capacities for school improvement, in five more successful and four less successful Swedish secondary schools, and compare the qualitative similarities and/or differences in the principals´ views at the level of schools. A successful school is understood to be a school where pupils accomplish both the academic objectives and the social/civic objectives in the National Curriculum. The empirical materials used were collected through semi-structured interviews with the principals and deputy principals, and through general school observations in the nine schools. The perspective of principals´ views was used as the unit for analysis, in order to reflect the principals´ way of thinking about the internal capacities, as principals´ views were expected to be an important indication of how principals act and interact with teachers in their specific context. To create such a model for analysis meant creating views, generated from empirical text, that deepened the understanding of the meaning of collaboration forms, staff development, and leadership, as critical internal capacities for school improvement. These views were then interpreted with the help of two theoretical concepts; structure and culture. The creation of the model made it possible to analyse and describe the school observations and the principals´ views of the three critical internal capacities, in the same usage. The question of what is decided helped to describe and understand the structure in a school, which in educational sociology is understood as the division of labour. The question of how the decisions are realised helped to describe and understand the culture in a school, the distribution of work. By using the theoretical concepts of structure and culture it was possible to unfold the power relations and the modes of control in the schools, regarding the three internal capacities for school improvement. One part of the result was the constructed view types for collaboration forms, staff development and leadership. It was possible to construct three qualitatively different view types: A principal distributed and team-based/involving view type, a principal distributed and teacher-based/traditional view type, and a politically distributed and principal-based view type. The last view type only appears in relation to staff development. When connecting the principals´ views of the three internal capacities in the different schools to the different view types, the results show that the ´team-based` view type dominates in all of the more successful schools, as well as in one of the less successful schools. In two of the less successful schools the ´team-based´ view type has become a vision for the principals to strive for in relation to the experienced reality of the ´teacher-based´ view type. The remaining less successful school is dominated by the ´teacher-based´ view type. Principals´ views of external collaboration forms, the connections with the world outside the school-house, are interesting, as all schools no matter the level of success, are ´teacher-based´.