Educating children with disabilities alongside their peers in mainstream preschools has increased intensely over the past few years, affecting all aspects of early childhood education. Many children who previously would have been educated in segregated special centres are now being included in inclusive preschools. This research paper discusses how Thai preschool teachers' professional knowledge in inclusive education influence the ways they practice within preschool classrooms. Qualitative data obtained through observations and collaborative inquiry with teachers drawn from four preschool in Bangkok, Thailand showed that the lack of adequate teacher preparation for inclusive practice rendered the teachers helpless and unable to deliver curriculum that caters for the learning needs of children with disabilities. This paper contributes to the growing body of research to inform the usefulness of collaborative inquiry in helping teachers explore educational issues in systematic ways and determine solutions through reflection and dialogic inquiry.