This research aimed to describe pre-service EFL teachers’ understanding of reinforcement, the types, the purpose, and how reinforcement is delivered during teaching practicum. A qualitative approach with a descriptive qualitative design was employed. The data collection techniques were through observations and interviews with three pre-service EFL teachers from one of the state universities in Indonesia majoring in the English Education Department. They were purposefully selected. Additionally, this research also used the teachers’ and students’ documents. The data were validated by triangulation and analyzed by following the interactive model analysis. The reinforcement theory propounded by B.F Skinner analyzed the teachers’ reinforcement.

Based on the data analyzed, it was found that only pre-service teacher provided relatively the same definitions as the experts, pre-service teacher 2 perceived reinforcement as the encouragement for students to do something better during the teaching and learning process. However, two pre-service teachers have a different understanding from the experts and the theory. They use positive reinforcement 96.57 % in the form of social reinforcement (69.14 %), tokens (24%), and tangible reinforcement (3.43 %). In addition, there are no pre-service teachers who use activity reinforcement. In the other hand, they used 3.43% negative reinforcement of the total reinforcement used in the class. The ultimate goals of positive and negative reinforcement used by pre-service teachers are to motivate and encourage students to repeat the expected behavior. The researcher discovers that pre-service teachers employ reinforcement for other purposes, such as to instil students’ confidence, to foster a relationship between the students and the teacher, to make students feel happy and appreciated, and to motivate students to participate in the course actively. There are seven ways to deliver reinforcement based on Skinner’s theory. The results indicate that one of the three pre-service teachers provided six ways of reinforcement throughout their teaching practicum. In contrast, the other pre-service teachers reinforce the students in three ways. It can be said that pre-service 2 has understood and implemented reinforcement during the teaching practicum. Besides, pre-service teachers 1 and 3 do not understand reinforcement but practically implement it. The finding of this study could be used as consideration in designing and planning the preparation course for teaching practicum in order to provide the necessary support for pre-service teachers to master the essential teaching skills, including reinforcement skill.