Growing awareness of the importance of education in the early years has led professional organizations and policy makers to consider the effectiveness of mathematics education for young children. Factors such as educational environments, early years curricula and teacher education are some of the many aspects that have been examined. This paper describes the types of mathematical content knowledge that 25 prospective primary teachers developed as they designed, taught and reflected upon early number lessons. This Lesson Study approach involved two mathematics educators and 25 prospective primary teachers designing, teaching and re-teaching five early number lessons in two elementary classrooms with 4-5 year old children. The results suggest that effective teaching of early number concepts draws on mathematical knowledge that is specialized and unique to teachers; in other words Specialized Content Knowledge (SCK). The study found that engaging in Lesson Study promoted mathematics SCK development in two primary ways, notable by: (1) raising awareness of the complex relationships between early number concepts that contribute to developing robust early number understandings and (2) developing expertise in identifying the nature and source of children's mathematical errors. The development of participants' early number SCK is described and recommendations are made for the design of experiences in Initial Teacher Education (ITE) to support teachers in the provision of high quality mathematics experiences to young learners.