The gilet is a sleeveless, windproof top, and its role is to keep the wind-chill off your chest, without the weight, bulk or superfluous warmth of a jacket. You can wear one over a short-sleeved jersey - with arm warmers - for cool starts or Alpine descents in summer, or over a long-sleeved jersey in winter.
And when you don't need it, it'll scrunch up to fit in a jersey pocket. If you keep moving briskly, a gilet will also keep you warm enough - if not necessarily dry - when the weather turns damp. Few, if any, gilets will shrug off sustained rain, but most are showerproof, and when you're wearing cycling gear underneath, that's enough.
Dark colours look cool but it's safer if you're conspicuous: yellow red orange and lime green all stand out. Reflective patches or piping are also useful at night.
Gilets are usually made from lightweight polyester or nylon for the windproof front and shoulders. To increase breathability, the sides are often mesh. A number of gilets use a microporous membrane for greater wind/rain protection and thin fleece may line the collar.
Better rain protection from a gilet can be had from a water-repellent surface treatment a polyurethane (PU) layer, or an ePTFE membrane, as well as taped seams. A storm flap for the front zip will also help to keep both wind and rain at bay. Design and fit To prevent flapping fabric and draughts, look for a close cut with an elasticated hem, armholes that aren't too wide - cyclists aren't bodybuilders. The collar should be high and close to keep your neck warm and not scoop air. Ideally the front zip should be operable on the move; look for a pull tag.
Weight & Bulk
Gilets score over other layers by being so compact - even when you factor in arm warmers. Most gilets will fit in a jersey pocket. The easier a gilet is to carry, the more often you'll have it with you. Pockets These aren't essential if you've got pockets underneath, but are nevertheless handy for your backdoor key, phone or arm warmers.
Most gilets can be machine washed on a cool setting (30 to 40º). Most can't be ironed or washed with fabric conditioner, although some can be gently tumble-dried. Always check the label.