Professionalism for language teacher trainers and teacher educators would require purposeful preparation, induction, and support to develop the knowledge base necessary to foster teachers' development. This study seeks to learn more about the current state of TESOL teacher trainers' professional development in the United States, including their preparation, expertise, and recommendations to improve the professional development of TESOL teacher trainers in the future. It found that teacher trainers in many contexts receive limited formal preparation, induction, and support. Instead, they must independently identify professional development opportunities. Perhaps due to this lack of access to formal professional training, trainers had limited awareness of the professional knowledge base of the language teacher education field. Instead, they reported relying on personal traits and hard work as they gain expertise through on-the-job learning. This study's implications suggest that formal preparation, induction, and support programs could help teacher trainers and educators more effectively foster the learning and development of the teachers they work with, and could strengthen the professionalism of teacher educators, trainers, and the TESOL field as a whole.