Cooler in the heat.warmer in the cold, stays drier, most durable and best fit.64% merino wool, 28% nylon and 8% elasticSmall - women's 6-8, men's 4.5-6.5Medium - women's 8.5-10.5, men's 7-9Large - women's 11-13, men's 9.5-11XL - men's 12+