The Leitner system is a powerful spaced repetition method that helps you memorize more effectively by reviewing difficult material more often and mastered items less frequently. Developed by Sebastian Leitner in 1972, it uses a simple box-based flashcard system to strengthen memory and boost long-term retention. This guide explains how the system works, why it’s so effective, and where you can use it online through platforms like Anki, Quizlet, Brainscape, Memrise, and LanGeek.