This research is a grounded theory study that aims to generate a theory to account for Thai EFL pre-service teachers' materials development. The participants were twenty-two EFL pre-service teachers. Three rounds of stimulated recall interviews, consisting of five main research questions, were implemented. The resultant data was then analyzed using three coding processes: open, axial and selective coding. Peer debriefing and member checking were used to strengthen this qualitative research. The theory explains that Thai EFL preservice teachers' materials development processes comprise four main stages. The first stage is the pre-development stage. The process is affected by two main aspects: personnel and school. These aspects are influential in terms of opportunities, awareness, and regulation. The second stage is the development stage, which deals with developing materials. The third stage is the implementation stage, in which pre-service teachers analyze the materials and either implement them as planned, or with difficulty. In the final, post-implementation stage, they reflect on the suitability of their materials used in the classroom. The research demonstrates the professional development concerns of the pre-service teachers and has implications for the teaching profession community.