Although self-directed learning is considered important in both educational practice and theory as a 21st century skill and as a means to motivate students, most teachers find it difficult to integrate self-directed learning into their practice. An instrumental case study was conducted to investigate how teachers at four innovative Dutch secondary schools define self-directed learning and how they try to enhance it in their students. Special attention was paid to how the teachers dealt with differences in students' ability in self-directed learning. It was found that the teachers' definitions of self-directed learning varied from students working independently (which seemed closer to the concept of self-regulation) to students making decisions about learning goals and content. Methods used to enhance self-directed learning varied from clear instruction and well-organised learning materials to having students carry out self-designed projects. Teachers' ways of dealing with differences depended on their schools' interpretation of self-directed learning.