I'm a 36C and ordered a size M. This bra is perhaps the best sports bra I have ever owned. Unlike some styles, this really is a high impact, compression bra that will smoosh your girls tightly to prevent ANY movement. As a very active 27 year old who runs, snowboards, rock climbs, and cycles, this bra is everything I need. On my first wear, the bra did cut into my side a little under my armpit, but I didn't have any issues later on.

Note that this is not the bra you would want to wear outside of your exercise routine since, due to its great compression, will make you look at least one-two cup sizes smaller.