While all Castelli jerseys are designed to fit snugly; the Aero is designed to stick to your skin. It's a skinsuit of a jersey, saving you 10 watts compared to a normal jersey. That's almost a minute saved over 40k at 40kph. That kind of savings is a pretty heady promise. Thankfully, it is much more comfortable than a traditional spandex skinsuit top. That's due to the ultra-thin Velocity fabric in front and on the shoulders, and the mesh panel in back. Velocity is thin, light, breathes well and stretches well. The jersey in your hand feels incredibly small. Your shoulder sockets and upper arms, when the jersey fits properly, seem to be bursting through the sleeves. The mesh panel takes in about half your from mid-shoulder blades on down. The pockets are mesh as well. The arms have Giro++ grippers at the end, which is a thin single layer of stretchy fabric that holds onto your arm without silicone and without pinching. The grippers are flat, and quite naturally, fast.