Spaced repetition reviews material at increasing intervals (1 day, 3 days, 1 week, 2 weeks) rather than massing it together. This produces 2–5x better long-term retention for the same total study time. Use Anki (free, digital, automatic scheduling) or the Leitner System (physical flashcard boxes). Best for: vocabulary, programming syntax, formulas, definitions, historical facts. Not a comprehension tool — use it to cement what you’ve already understood. 15 minutes of daily Anki review produces dramatically better retention than any amount of re-reading.