The thesis presents an investigation into the professional development processes engaged in by lecturers who have taught English as a Foreign Language (EFL) in Thailand's Rajabhat universities. The aim of the study was to explore the Rajabhat EFL lecturers' learning processes, and examine how they related to their classroom practice. In particular, the study looked at: 1. how these lecturers acquired and developed their professional knowledge during their careers; 2. how they integrated their professional knowledge into classroom practice; and 3. the impact of professional development on their classroom practice. Historically, the professional development of the Rajabhat EFL lecturers has been predominantly formal, neglecting the informal mode of learning. Furthermore, teachers' professional learning from the classroom has been disregarded. This study attempts to clarify the roles of the two modes of teachers' learning and of classroom practice. The aim of the study was to explore the Rajabhat EFL lecturers' learning processes, and examine how they related to their classroom practice. The study employed a case study design with a qualitative, ethnographic style approach. Three selected experienced EFL lecturers were the research participants. Data collection methods consisted of life history interviews, classroom observations, semi-structured and student group interviews. Data analysis was based on the emic approach. The findings revealed a significant and continual interplay of three elements: the institutional context, the individual teacher, and the teacher's classroom in generating professional development, and thereby, the professional knowledge of the EFL lecturers. Knowledge is acquired and reinforced from both formal and informal professional development but less so from their practice. This was because their meaning perspectives have been focussed less on the students' context, thus addressing insufficiently the impact of their teaching on students' learning. This study provides a significant contribution to academic development in the area of Teaching English as a Foreign Language (TEFL) in the Thai higher education context. Formal and informal learning, the teachers' personal context, and the role of the classroom as a resource for professional learning all contribute to the development of professional knowledge for these lecturers.