Midlayer or standalone, the Atom Vest is a proven performer across a range of seasons, landscapes and activities. With its most advanced insulation, fabrics, and design ever, this evolution builds on its legendary performance. Coreloft™ Compact insulation retains warmth if wet and withstands years of packing and unpacking, and the breathable, durable Tyono™ 20 shell resists wind and light weather. Stretch fleece side panels aid ventilation and freedom, and the entire vest packs into its own pocket. Size and Fit Tips: Fitted Fit - This trimmer fit is patterned to stay close to the body through the chest, waist, hip, and thigh. Its low profile eliminates bulk, moves with the body, and fits comfortably over our first layers.