Elucidating the knowledge base of those who teach adult learners in ESL (English as a Second Language) programs, this research is a qualitative study incorporating multiple data collection techniques and involving practitioners with various backgrounds from different program settings. Although educational opportunities targeting adult ESL students play an essential role in equal and extensive participation in society and academia, the status of teachers who assist these learners achieve their goals is rather marginalized, often because the students themselves are marginalized and ESL teaching is considered an undertaking that does not require special preparation. One way to enhance the professionalization of those involved in ESL is through an explication that the task of providing instruction to ESL learners is indeed an endeavor filled with complexity and requiring a strong knowledge base. This study was an attempt to contribute to this effort in fostering such professional recognition. Data were collected via a series of focused interviews, consecutive classroom observations, and a stimulated recall procedure with each of 10 teacher participants. Analysis of the data revealed that ESL teachers possessed an intricate knowledge base with multiple categories of knowledge that they called upon to deliver instruction effectively and efficiently, particularly through the management of student responses and the management of learning. The interconnection among the seemingly discrete knowledge categories further highlighted the complexity and difficulty involved in the provision of instruction to adult ESL learners. By comparing the teacher participants’ instructional effectiveness and various backgrounds, the study also illuminated the impact of ESL teachers’ professional preparation and language learning experience. Implications derived from the findings are offered for theorists and researchers, and for practitioners and administrators of programs that serve adult ESL learners.