I wear an 8 1/2 medium and ordered the size 39. It fits very well, with plenty of room in the toe box and a secure heel grip. The toe box is wide enough to fit comfortably with no bunching or pinching. I was nervous about having non-adjustable straps on top because of my high arch, but it isn't an issue. I tried the shoes on both in the morning and in the late afternoon, and they fit well at both times. So far the only thing I'm not totally thrilled with is the stitching on the top straps. It does stand out from the shoe, and I'd prefer it to be less noticeable. Overall, though, this is a cute, comfortable shoe that can be worn with either jeans or pants. I wanted something to replace standard tennis shoes and be more stylish, and this shoe didn't disappoint me.