I purchased an 11 and normally I wear a 10 1/2, but since that wasn't available, this worked great.  I would say they run on the small side since the 11 is OK, not too big.  I had Rika pool shoes prior to these.  THese don't have the support built in that the Rika did, but they work.  I wear these 5 days a week for aqua pool classes.  I had a little problem with the left shoe rubbing the edge of my foot, so I put a pair of short tennis socks on and then it was fine.