@NaturalGirl , thank you for reaching out! This is a great question. While cycling and running socks can look similar and are often made with similar materials such as merino wool or a polyester/spandex blend, the biggest difference between a cycling sock and a running sock is going to be the cushioning. Running socks, such as the PhD ultra-light micro socks are designed with high density cushioning due to the repeated impact of an activity like running. Cycling socks are designed with very minimal cushioning. Cycling socks will often have a seamless design in the toe to prevent chafing. Happy Adventuring!