Repetition-induced memorization is more solid when the occurrences of a given piece of information are spaced apart in time (i.e., distributed) than when they are not spaced (i.e. massed). We propose a theoretical synthesis of the effect of distributed practice in light of the recent work in experimental psychology and cognitive sciences. The deficient processing hypothesis is the most compelling to account for the memory deficit entailed by massed repetitions. The study-phase retrieval hypothesis seems the most suited to explain the effects caused by the variation of the spacing between repetitions. The encoding variability hypothesis, although largely cited in the literature, does not appear satisfactory. We discuss new approaches, such as memory consolidation and the role of sleep and evoke educational implications.