The spacing effect (also known as distributed practice) refers to the finding that two or more learning opportunities that are spaced apart, or distributed, in time produce better learning than the same opportunities that occur in close succession. A number of theories have been proposed to account for the spacing effect. These include deficient processing, encoding variability, study-phase retrieval, and consolidation. According to the deficient processing account, learning opportunities that are spaced apart in time, compared to non-spaced or “massed” learning opportunities, are more likely to receive a learner’s full attention, ultimately leading to better quality learning. The encoding variability account proposes that spaced learning opportunities, because they are separated in time, are more likely to be associated with a number of different contextual cues that can benefit later memory for the information learned. Study-phase retrieval is based on the premise that retrieval benefits learning, and spaced learning opportunities are more likely than massed learning opportunities to involve retrieval of the previous learning experience. More recent evidence suggests that spacing learning opportunities across different days may benefit memory due to sleep-dependent neural consolidation processes. Research in authentic educational contexts shows that spacing benefits learning of a wide variety of materials, from basic facts to complex scientific concepts and skills. Regarding the practical question of when spaced learning opportunities should occur, the ideal scheduling of these opportunities depends upon how long the information needs to be remembered in the future, such that retention over longer intervals of time benefits most by longer spacing between repeated learning opportunities. Despite its promise for enhancing student learning, spacing can be challenging to implement in authentic educational contexts due to the intuitive notion that immediate repetition is better for learning, and the difficulties involved in setting a spaced study schedule in advance and adhering to it. To realize the full potential of spacing to enhance educational practices, future studies are needed that can measure implementation of spacing by students and teachers in real educational environments.