This not my first pair, so I knew exactly what I was getting.  I normally wear an 8.5 and according to the size charts I should wear a 39, but they are too big for me and I need a 38.  I have slightly wider than average feet and order the M width and they fit me just fine.  I do not have any problem with the toe posts as I read some other reviewers do.  It is a very snug fit, so I can see where some may have issues.  I suffer from P.F., and my cheap flip flops with no arch support are to blame.  I have also noticed that since I started wearing Birkenstocks during the warmer weather, I don't have the ankle, knee, and hip pain I had before.  And pain from p.f. is gone!  I ordered these on a Thursday, chose next day shipping and they were here before noon on Friday.