I did a lot of investigating before I purchased this shoe.  I think it's best to visit an actual store and try on the Birkenstock brand before purchasing online.  With that said, I wear a size 7 and ordered a size 37 in Birkenstocks.  Others have commented that the strap doesn't fit around their foot like they wish, and while the strap, indeed, doesn't stay high on my ankle, it does keep it on my foot without my having to hold the shoe on with my toes, which was why I wanted this particular pair.  Birkenstock is a fantastic and very durable brand that is well worth the money.  As a school teacher who rarely sits down, I couldn't ask for any better footwear.