Swiftwick is an amazing company with fantastic customer service. My shoe size of 9.5 is the upper limit for the medium size socks. I had purchased numerous medium size pairs and loved them initially, but had some subsequent difficulties. I notified, Swiftwick Support, not expecting anything in return. However, they went so far above and beyond my expectations And replaced my socks with the larger size. I have never experienced such great and responsive customer care. Swiftwick is my favorite running sock by far (in particular the Aspire 7). With their great products and great customer service, I will be a customer for life.