Breathability is probably the biggest concern outdoor enthusiasts have about garments. Back in the day, fleece wasn't very breathable, but with modern innovations that have produced "micro-fibers," fleece now rates as one of the most breathable products out there. Manufacturers have also created new designs that utilize lighter-weight fabrics in key heat-producing places, like armpits, to increase breathability while using thicker fleece materials to keep your core warm. Don't mistake breathability for windproofness, however. Unless your jacket has a breathable inner or outer layer that stops the wind, fleece by itself does not protect well against the wind. On windy days, wear a lightweight windbreaker on top and you'll be all set.