For extremely warm weather, Heat Gear pioneered the revolution of UA's "second skin" design which gives you the support you need without restricing your movement. The high performance line fits tight on the body like you are wearing nothing at all. This Under Armour shirt will look, feel and perform unlike any other shirt you have ever worn. The V-Neck provides all the benefits of the HEATGEAR Full T, but is designed for reliable moisture transport and comfort under the uniform or vest. Sizing chart: http://www.underarmour.com/mens_size_chart.htm (Cut and paste in a new screen on your web browser.)