This thesis reports on an international multiple-case study on innovation in upper secondary schools. A total of four secondary schools are studied: two in Norway and two in Germany. A mixed method with quantitative and qualitative parts is applied. The quantitative part consists of a survey of relevant teachers at the case schools. The qualitative part includes the study of school documents and secondary literature as well as visits to the schools and interviews with key personnel. All subjects and participants are anonymised. The study aims to portray innovative practices in schools and to identify reasons why some initiatives for change are more successful than others. It turns out that innovation processes in schools are often complicated and lengthy; many things that can hinder the implementation and continuation of an innovation. This work is valuable to anyone who is interested in school development, either people who want to develop schools or those affected by school development.