I ordered the Small in the Icebreaker GT Run Rush Sports Bra and found it too small - I could barely put it one and ended up tight in the bust and way too tight around my chest (at the base). I am waiting for Sierra to bring in other sizes as I really need a Medium. In a regular bra, I take a 36C and have managed with a B cup. The most important for myself is the circumference 36, otherwise too tight. Hope this helps.