I'm a 36D so I ordered a large. Its fits decently well. I think a medium would have been too tight for me (I bought it to mostly wear as a top rather than an actual bra) but there does seem to be a little bit of a bubble of extra cloth as the top goes under your armpit. I'm not sure if this is from my breasts being too large and preventing the fabric to lay flat or if my breasts are not large enough to completely fill it out. Either way, the top is comfortable is incredibly cute so I am still happy with the purchase!
Megan recommended this style