Layering is the practice of wearing multiple garments to create a temperature-regulating system. The three-layer rule: base (moisture management), mid (insulation), outer (protection from wind/rain). Base layer: merino, polypropylene, or moisture-wicking synthetic. Mid layer: fleece, down, or wool knit. Outer layer: waterproof/windproof shell. Effective layering reduces the total warmth needed from any single garment and allows real-time adjustment. Critical in variable climates.