English for specific purposes (ESP) has been conceptualized by its leading scholars, like Hutchinson and Waters (1987) or Dudley-Evans and St. John (1998), as a multi-stage process, where the ESP practitioner fulfils a variety of roles, including that of learner needs researcher, course designer, language instructor, learning assessor, and course evaluator. The performance of these roles requires considerable knowledge of a linguistic, socio-cultural and pedagogical nature, necessary to inform the teacher’s cognitive processes, par- ticularly those involved in course decision making. The necessary professional knowledge of the ESP teacher, which is gained through professional school- ing, teacher training, and teaching experience, comprises both relevant theo- retical concepts (knowing what) and performance skills (knowing how). It di- rectly impacts on all stages of the ESP process, namely the planning, design, teaching, assessment and evaluation of a course, largely determining its quality. The present paper focuses on ESP teacher cognition, especially those cognitions (i.e. knowledge and beliefs) that are involved in course design, informing the teacher’s choices of course parameters and instructional practices. Elaborating on the concepts developed by language cognition scholars, like Shulman (1987), Andrews (e.g. 2007), and Borg (e.g. 2006), the author tries to outline the in- ternal structure of ESP teacher cognition and describe the function of each subordinate knowledge base. The paper also presents the preliminary results of a small-scale exploratory study into the professional cognition of 13 teachers of Legal and Business English employed at the University of Warsaw.