Under Armour Boys' HeatGear Armour Up Digi Fitted Short Sleeve Shirt 90% Polyester, 10% Elastane Imported Where we started? It all started with an idea to build a superior T-shirt The technology behind Under Armour's diverse product assortment for men, women and youth is complex, but the program for reaping the benefits is simple: wear HeatGear when it's hot, ColdGear when it's cold, and AllSeasonGear between the extremes