Well, I'm a 36/38 DD and I got a medium and it actually keeps the girls in line.  I was shocked.  One is a little flimsy but if you wear two at at the same time it works great.  NO MORE UNDERWIRE! I would recommend getting a size smaller than you think you need and then wearing two at a time for best support.  But, be aware, one is kinda flimsey so you wouldn't want to wear it under anything shear like a tee shirt or (if you are busty like me) you look like you are not wearing a bra at all and nobody wants to see that.