Professional development is deemed necessary for university teachers at all levels, as it helps to enhance teaching quality. However, the extent of English language university teachers' professional development might depend on a number of factors. This paper reports on a study investigating English language university teachers' professional development in Thailand, the factors related to their development, the activities they used and the obstacles they encountered. The study was conducted through the use of a questionnaire to collect data from 217 full-time English language university teachers at 14 universities in Bangkok metropolis, selected through the multi-stage sampling technique. The results indicate that the overall mean score of Thai university teachers' professional development was at a moderate level, and the factors of gender, academic title, degree and job responsibility were not related to professional development. Only one factor, university type, was found to be related to their development. This means that private university teachers had more professional development than those in Rajabhat and government universities. Discussing or sharing knowledge with colleagues was the activity they used to develop themselves most, with heavy teaching loads being the main obstacle. In addition, students' background knowledge was the main obstacle to implementation of the received knowledge in teaching. (Contains 10 tables.)