The Response to Intervention model originated in the United States. The model aims to prevent learning difficulties through early identification and thus early intervention. The goal is that no student should fall behind “No child left behind”. The purpose of the thesis is to investigate how RtI can function as a special didactic model in primary school. It seeks to systematically organize early identification and early intervention regarding students' reading development and thereby counteract reading difficulties. The thesis first study is a meta-analysis that examines the effects of tier 2 interventions on primary school students' word decoding development. A small to moderate effect was found g = .31. The second study examined the effects of RtI when identifying and intervening students in need of support in their reading development. The study was conducted as a quasi-experiment with grade 2 students (n = 11 + 11). The results showed effect sizes between g = .49 - 1.00 on word decoding and reading comprehension, however, the results were not significant. The teachers' perception of the model was also compiled. They found that RtI worked very well as the students received support and the teachers were given the opportunity to collaborate. They did however also find that the RtI model was inflexible and resource-intensive. The third study was longitudinal and followed students' (n = 113) reading development during grade 1 and 2 within RtI. A significant reduction in students in need of support was noted. In comparison with a reference group (n = 759), there were significantly fewer students who performed below the 25th percentile and fewer RtI students who did not maintain their reading ability. The results showed that RtI seems to function as a special didactic model in accordance with "No child left behind". Nevertheless, there are students who have received interventions within the model that do not reach age-appropriate levels. The fourth article discusses the possibilities of combining Response to Intervention and Lesson Study as the models seem to be able to complement each other.