A cornerstone of inclusive education is teacher's readiness to respond adequately to different educational needs of students in their diverse classroom. Differentiated instruction, referring to the process by which teachers carefully monitor students' needs and progress, and adapt their instruction according to these differences, is a means to meet the needs of students of different levels. High-quality differentiated instruction is a complex process for teachers which needs careful preparation. This makes teachers' intentions relating to differentiated instruction particularly relevant. In this article we compare two theoretical models to explain teachers' intentions to differentiate: the Theory of Planned Behavior (TPB) and Self-determination theory (SDT). Secondary school teachers (n = 180) responded to a questionnaire in which we measured their intentions and predictor variables relating to both of the theoretical models. Linear regression showed that both the TPB as well as the SDT model significantly predicted teachers' intentions (p < 0.01), but that the TPB model showed a better fit with the data (48 versus 32% explained variance). We conclude that in addition to the factors included in the TPB, autonomy might be a relevant factor in explaining teachers' intentions.