If you are looking at this for use as a base layer, which means you want it to fit more tightly against your skin to wick sweat, I would recommend going down one size.I normally wear a Large in most tops. I ordered a Large and found it was big enough to wear as a second layer, like over another shirt. I then ordered a Medium and it was perfect. Much closer to what I was looking for as a base layer, but not skin tight like a compression shirt. And it was still plenty long on the torso and just long enough on the sleeves. Great purchase.