I ordered this in a medium based on the size chart included on the product page which only asked for the chest measurement, not the cup size.  So, according to that, I needed a medium.  When it came, the tag on it said the sizing was for a 34A up to a 34B.  I'm a usually a large 34B to a 34C.  It looked big enough, so I tried it on.  It was kinda hard to get on, and I uh chaffed a delicate part of my front half getting it on.  It still seemed to hold and fit ok, and even gapped a little around the arm pit area, so I thought maybe the fit won't be too small despite the ominous tag sizing.  Well, then I tried getting it off, and it was extremely difficult.  And I wasn't even sweaty yet.  I can't imagine how this thing is going to come off once I am all sweaty post Zumba. Not sure if that just means it's actually too small as the label seemed to suggest, or just the nature of the bra in general.  but, the fact that it gapped a little under the armpits seems to show that the size shouldn't be the issue at hand.  Other than that, once on it felt comfy, and supported well and kept pancake b00bs from occurring.  I'll try it during my Saturday yoga and zumba workouts and update this review if there is anything to add once I've tried it in action.
-----------UPDATE-------------------------
I wore this for a yoga class followed immediately by a zumba class.  I wore it underneath a basic ribbed tank.  The bra supported very well and looked really nice under the tank! I still looked like I had a chest!  The top mesh portion where the piping is would not lay flat, though.  It didn't affect support, but looked a little funny. Not sure if I needed to tighten the straps, or if its too big/too small, or just a flaw.  I did find that after it was loosened up by my workout, it really wasn't as bad to take off as was the first time I tried it on.  For looks and support,and the fact that it did loosen up a little, I bumped my review up to 4 stars. I will also update how it stands up in the wash once I've worn and washed it a few times :)