You should purchase the size that most closely corresponds to the size of shoe you most often purchase that fits well. For example, if you've purchased four pairs of shoes, three of which are a Women's 9 and the final one of which is a Women's 9.5 and all four pairs fit well, we'd suggest purchasing the size Women's 9 in the item you're interested in ordering. If you're between sizes, we typically recommend sizing up to the nearest size, but many of our footwear products state whether the product runs large or small to help you make a decision on sizing up vs. sizing down where necessary.