Background: Education in mathematics and science is important at all school levels and should ideally start in the preschool, which implies that preschool teachers should have an adequate knowledge base in these subjects.

Aim: This study aimed to investigate preschool teachers’ knowledge bases, including their subject matter knowledge (SMK) and pedagogical content knowledge (PCK) in mathematics and science.

Setting: The study presents an account of preschool teachers’ knowledge bases in mathematics and science in Botswana and Sweden and how they influence their teaching choices and strategies.

Methods: A pragmatic paradigm with a survey design and a mixed-methods mode with quantitative and qualitative approaches was used. Using a purposive sampling procedure, 64 preschool teachers from Botswana and 67 from Sweden participated in the study. Questionnaires and observation schedules were used to collect the data.

Results: The findings showed that the Swedish preschool teachers had SMK and PCK in the two subjects. The Botswana teachers had SMK but lacked PCK. The early childhood educated preschool teachers from Sweden with developed PCK highlighted the daily routine situations as teaching or learning situations, whereas the primary school teachers from Botswana had SMK but found it difficult to teach at the preschool level.

Conclusion: The preschool teachers’ knowledge bases included a play-based approach and knowledge about didactic considerations in spontaneous situations that made them teachable. The findings also showed the importance of a framework for specialised and professional early childhood education (ECE) training and acquiring SMK and PCK that contributed to the preschool teachers’ knowledge bases and practices in their teaching of mathematics and science. Early childhood education teachers’ educational experiences, their specialised and professional training profiles and the knowledge they acquired determined their knowledge bases and the SMK and PCK that minimised the constraints and challenges faced by them in the teaching of mathematics and science in ECE classrooms.