Based on the countless other reviews that said this jacket ran large I decided to go down a size and got a medium.  Once I returned that and ordered a large I loved this windbreaker.  For those of you who are like me and are bit well endowed in the chest (I'm a 38D) going down a size may not work for you.  I could hardly get the medium to zip up over the girls with just a shirt on (let alone a hoodie or multiple layers).  In my experience ordering the size you normally would works out just fine.  The large allows me to put a hoodie underneath, which is great because I plan on wearing this while sailing and this jacket is really thin.  I'd definitely recommend.