Great - Order Large

I have about 5-6 Under Armour shirts and these have been my favorites. While I love this shirt, I think they have terrible sizing description.I am 5'8" 155 lbs, decently athletic. I usually wear small > medium size shirts.I order an Large for this shirt.It can be really tight in the armpits and I've nearly gotten stuck in small and medium size shirts (literally stuck). So, order a size or two larger than you think you should wear. I have put on an XL and it still felt pretty good, but Large does fit me *slightly* better, so -- order up one size, at least.