It almost doesn't seem possible that a vest this lightweight can perform this well. When you first pick it up, you almost want to toss it in the air it's so light. Castelli FLY Fabric Technologies: FFT make this almost-impossible vest possible. The CLIMA SL insulation layer and FLY SL shell are made with 100% recycled fibers that are amazingly soft and lightweight. The FLY SL shell is also treated with a water-repellent finish to keep you dry. The GORE-TEX INFINIUM™ WINDSTOPPER® inner lining provides outstanding wind protection and is extremely breathable. The back of the vest has a stretchy jersey-weight fabric to reduce the weight and allow for a close, comfortable fit. The low collar sits against the chest to keep any wind from coming in, and the YKK® Vislon® zipper slides easily to open up the vest for extra ventilation. Three back pockets allow for ample storage, and reflective details increase visibility. It's amazing this much protection and practicality can be found in such a lightweight vest.