The aim of this article is to describe teachers’ and principal’s reflections on development needs in mathematics and science teaching in a Swedish lower secondary school, in grades 7–9 (13- to 15-year-olds). Furthermore, it is to enlighten and discuss the challenges in the transition between identified development needs, decisions on measures and implementation of them. Eight teachers and their principal participated. Data were collected through focus-group interviews and individual semi-structured interviews. The expressed development needs were at external and internal levels. The participants prioritized the needs at the internal level, within the school, with focus on aspects of assessment, student understanding, instructional strategies in mathematics and science as well as science curricula. Our findings indicated that changes in organization and leadership, a reduced motivation among teachers, deteriorated work environment and insufficient external support were barriers when deciding measures concerning development needs, and implementation of them. The study illustrates the complexity and challenges in efforts to develop a teaching practice in a Swedish context.