An ever-increasing number of children with and without disabilities are attending early childhood programmes and learning together. Early childhood inclusion considers all children with and without disabilities, and their families as full members of the early childhood community. Although many early childhood teachers accept the educational rights of children with disabilities and the core principle of inclusion -- that early childhood learning programmes should provide for the needs of all the children in their centres, regardless of ability and disability, there remain significant barriers in terms of teacher professional knowledge in achieving these goals. In this article, we report a study on Thai preschool teachers' knowledge of inclusive early childhood education. Quantitative data obtained through a questionnaire were supplemented by teacher interviews. Both the quantitative and qualitative data focused on the teachers' assessment of their professional knowledge. Findings from this study can inform effective professional development programmes in preparing early childhood teachers for successful inclusive practices. (author abstract)