Cooperative learning is an evidence-based teaching strategy. In cooperative learning, teachers structure students' interactions and prepare them for cooperation so that students work together in small groups supporting each other's' learning processes. This study investigated whether the empirical evidence of the effectiveness of cooperative learning is reflected in teachers' professional competencies and their teaching practices. We surveyed 1,495 language teachers in Poland, measuring their knowledge and beliefs about cooperative learning and their use of cooperative learning in class. Although teachers were well informed about the principles of cooperative learning, they only knew a few methods to implement cooperative learning in class. Teachers agreed that cooperative learning is effective for students' academic and social learning and can provide students with individualised support for their learning processes. Despite these positive beliefs, teachers used cooperative learning infrequently. When they used cooperative learning, teachers organised and supported students' interactions in accordance with the principles of cooperative learning. Teachers reported that they would like to learn more about cooperative learning and use it more often in class. They were especially interested in support such as lesson examples and teaching materials. We discuss the implications of these results for teacher education.