This paper presents the findings of a qualitative study investigating which factors inside and outside the classroom result in feelings of stress for TESOL teachers working at private language schools in South Africa. Using in-depth semi-structured interviews, the findings reveal three main areas that cause stress for TESOL teachers: the job of teaching, relationships at work and organisational and TESOL-related issues. These three areas of stress can be further divided into various sub-themes. Stress resulting from the job of teaching includes the sub-themes of work overload, inadequate training and gaps in subject knowledge, time pressure and student behaviour. Stress resulting from relationships at work includes the sub-themes of negative relationships with colleagues, administrators and management. Stress resulting from organisational and TESOL-related issues includes the sub-themes of working conditions, lack of teaching resources, shared spaces and lack of professional development and career advancement. There has not yet been an investigation into the causes of stress among South African TESOL teachers. This paper is thus an attempt at laying the foundation for further research. The results provide valuable information for TESOL teacher trainers, TESOL co-ordinators and the management of private language schools.