The importance of number sense lies in its relationship with mathematical proficiency. Consequently, number sense is increasingly emphasised in curriculum documentation related to mathematics. The Primary School Curriculum: Mathematics (Government of Ireland, 1999b) is no exception. It implicitly stresses the importance of number sense and draws teachers’ attention to a range of key aspects. My study is premised on the belief that young children’s number sense is developed as a result of their everyday experiences with numbers, and these experiences form the basis on which further development of number sense, both in and out of school, takes place. Two questions are investigated: What number sense do young children demonstrate on entry to school?; What is the congruence between young children’s number sense and the curriculum for the first year of primary school? The first question is explored through individual experience-based flexible, focused interviews with fourteen children (eight boys and six girls) early in their first term at primary school. The age-range of the children was 4 years, 1 month to 5 years, 1 month. The second question is addressed by means of an analysis of relevant curriculum documentation and then linking that analysis to the data analysis. A framework for exploring number sense in young children was developed. This included four aspects: Pleasure and interest in number; Understandings of the purposes of number; Quantitative thinking; and Awareness/Understanding of numerals. Affective issues emerged as centrally important in the development of children’s number sense. Counting purposes were most transparent for children, with generic, communicative and label purposes also identified by them. There were striking differences in the ways in which girls and boys constructed their metacognitive framework in relation to number. Children’s vi Elizabeth Dunphy R 5148637 responses to quantification tasks provided interesting insights into how children perceived such tasks. The process of estimation appears to be challenging for many four-year-old children. A number of areas of omission were identified in the teacher guidelines that accompany the curriculum documents. Implications were drawn with respect to how aspects of children’s number sense, for example estimation, are developed at school. Implications for teacher education are also discussed.