Updated alpha sizing!.Under Armour’s mission is to make all athletes better through passion, design and the relentless pursuit of innovation.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.