Spaced repetition reviews material at increasing intervals rather than massing it together, producing 2–5x better long-term retention for the same study time. Use Anki (free, automatic scheduling — rate each card and it schedules the next review) or the Leitner System (physical flashcard boxes, Box 1=daily, Box 2=every 3 days, etc.). Best for vocabulary, code syntax, formulas, definitions, and any discrete fact needing fast recall. 15 minutes of daily Anki review beats any amount of re-reading.