Most cotton socks like the novelty socks we sell here at Cute But Crazy Socks should be washed after each wearing. That is because each time you wear them, sweat from the more than 125,000 sweat glands you have on each foot (a higher concentration than anywhere else on the body) works its way into your socks. If it isn't washed out, bacteria and fungi can grow rapidly in the warm, moist environment.The same is true when it comes to washing athletic socks which are made of mostly synthetic materials like our running and biking socks from the SockGuy brand. These socks wick away moisture and sweat, but when the sweat evaporates it can leave deposits on the socks that need to be rinsed out regularly, or they will weaken the socks' elastic.For wool socks like the ones from the Darn Tough brand, it is OK to wait a little longer between washes. These socks are made from from merino wool which has natural anti-microbial, anti-stink properties. Darn Tough recommends washing their merino wool socks after 2 to 3 full days of wear, but some may choose to wear them even longer.