Natural fibres come from plants (cellulosic: cotton, linen, hemp, ramie) or animals (protein: wool, silk, cashmere, alpaca). They generally offer higher biodegradability, moisture regain, and breathability than synthetics. Sustainability rating: plant fibres typically score 1-2 (best); protein fibres 1-2 (water use and animal welfare caveats). Natural fibres dominate in fashion's premium and sustainable segments.