Love, love, love these socks. They are super breathable, never slide with tennis shoes, are not visible when wearing tennis shoes, and are very durable. Can't say enough good things about them! They bunch up a tad at the heel when I first put them on, as a few other reviewers have noted, but I just jiggle the shoe up and down a couple times and then they slide right into place. I got the Medium and am a size 8 narrow shoe.