In mathematics education, APOS Theory is a framework of how mathematical concepts are learned. APOS Theory was developed by Ed Dubinsky and others and is based on Jean Piaget's notion of reflective abstraction. APOS stands for Actions, Processes, Objects, Schemas, the four main mental structures involved in the theory. APOS Theory takes a constructivist view towards mathematical learning. Implementations of APOS Theory in classrooms typically use the ACE Teaching Cycle, a pedagogical strategy with three chronological components: activities, classroom discussion, and exercises. Implementations also often use mathematical programming languages, most commonly ISETL.

APOS Theory emphasizes four mental structures: Actions, Processes, Objects, and Schemas and its mental mechanisms: Interiorization, Encapsulation, Coordination, Reversal, among others.

The mental structures are not necessarily gone through in a linear progression.