Size US11M and large fits me perfect. But the 1/4 large is bigger (too big for me to use for an event - the cushion doesn't align with the forefoot and heel). These socks take care and attention to fit correctly, but once fitted and placed properly they are very secure and have almost no movement next to skin. They are thin, and depending on the activity the potential for friction with the shoe must be considered with the specific shoe and shoe size/fit and tested before using in an event. Great for road runs, and I'm also impressed with their performance on trail at 30-40km 1000-2000m elevation, wet, dry, steep downhill, all good. Multiple train runs at those distances with minimal wear, I have complete confidence in them at those distances, wet or dry, no friction, no heat and no blisters, better than my experience with merino. Also best compression I've ever had in a sock.