I was told about these "socks" after my experience on the West Yellowstone/Old Faithful ride, where my feet got very wet and very cold. I bought a pair of the hyperlight, but found that they weren't warm enough for 35F days and colder. I went back and bought the mid-weight and last week used the "thicker" socks. They worked pretty good...my feet get really cold. But I am happy with them. I have a wide foot, with a high instep and tend to go with a size 7 shoe...6.5 if I can get a wide width, sometimes even a 7.5 size...I got the small socks....They are tight to get on because they don't stretch too much, but I don't think I would want the medium because then I would have "too much sock" left over....