The aim of this paper is to discuss the knowledge preschool teachers need when they teach mathematics. The study focuses on how the knowledge from curriculum affect and is transmitted to teaching and how content is selected, valued and organised when they work with mathematics and documentation. Earlier research shows that the official curricula are important and have impact on how teachers organise the practice (Carr, 2007; Clements & Sarama, 2007). This paper is discussing preschool teacher’s professional knowledge in relation to mathematics and documentation and curriculum. Curriculum theory is used as frame factor in the analysis (Dahlberg et al., 2005; Lundgren, 1972; National Agency, 2010 & 2012). The empirical data drawn from a larger study in Sweden (Bäckman, 2015) include interviews with 19 Swedish preschool teachers. Focus was on their understanding of knowledge and strategies in teaching mathematics. Analyses also consist of frame factors like policy documents and the national preschool curriculum. The ethical considerations follow the rules from The Swedish Research Council (2011). The main findings of the analyses concerning preschool teachers' professional knowledge in relation to teaching mathematics are that policy documents and the national curriculum are strong frame factors and also support different educational strategies. The teachers want to design learning environments that supports children’s mathematical exploring and learning. In that work they used documentation as a strategy to organise the teaching. Children’s teachers need knowledge of the frame factors, both international and national, when they create participatory practice and learning opportunities for children.