The base layer sits directly against the skin. Its primary function is moisture management: wicking sweat away from the skin surface to keep the wearer dry and regulate temperature. Best materials: merino wool (natural antimicrobial, soft, high moisture regain), polypropylene (fastest wick), or synthetic microfibre. Weight: 120-200 gsm. Key metric: low thermal resistance (CLO <0.5) to avoid overheating when active.