Fabric. Slings and strap-on carriers are made of comfortable brushed cotton, stretchy T-shirt cotton interlock, corduroy, polyester and polyester blends, flannel-like materials, or meshy, moisture-resistant nylon, and come in fashionable colors and patterns. Carriers designed to be used in water may be made of neoprene--wet suit material. Slings and strap-on carriers should be completely washable.

Musical amenities. The latest models of strap-on carriers play music from the front pouch and serenade your baby with five nursery melodies (you'll find that feature in the Evenflo Snugli 4-in-1 Serenade Soft Carrier, www.evenflo.com).

Cup or bottle holders. Cup holders are now de rigueur in cars, and some strap-on carriers feature this extra so you or your baby can sip anywhere.

Fasteners. Carriers have a variety of buckles and fasteners for shoulder and waist straps and babies' seats. Snaps should be sturdy and require a lot of force to unfasten. Buckles that hold shoulder and waist straps should be easy to adjust and keep straps firmly fastened when the carrier is in use. Buckles and fasteners should be easy for an adult to use, but not so easy that a baby could undo them, and should fasten tightly, but not close in way that they could pinch your fingers. If they do, your baby could be pinched, too. And you might not properly secure the buckles when using the carrier for fear of being pinched, which isn't safe either. Several carriers we tested, the Infantino 6 in One Rider, the Infantino Smart Rider, the Kelty K.I.D.S. Kangaroo, and the Snugli Comfort Vent by Evenflo, rated somewhat lower because the buckles posed a pinching concern for panelists (some did get pinched). (See our Ratings, available to .)

Lumbar support. Well-made carriers may have a special padded waist strap that helps distribute a baby's weight from your shoulders to your hips and pelvic area. This is a definite comfort advantage. The latest models of strap-on carriers feature a massaging lumbar support pad that vibrates, which is nice but not necessary. When you're shopping, try on a floor model and fasten the belt/waist strap to see if it's long enough and neither too high nor too low when the carrier is in place. Padding should be firm, not mushy. Our tests show that two carriers, the Baby Björn Active and the Jeep 2-in-1, have especially good back support.

Shoulder straps. Shoulder-strap padding should be firm and wide so the straps won't dig in. Straps should be positioned so they won't slip off your shoulders or chafe your neck, and they should be adjustable while you're carrying your baby.

Side-vent insets. Babies can get sweaty in an infant carrier. To help keep them cool, some carriers have a panel of meshy material designed to promote air flow, or side insets that can be unzipped or unbuttoned to serve the same purpose. Look for either of those features if you'll be having a summer baby or you live in a warm climate.

Side entry. Some strap-on models, such as the Snugli Classic from Evenflo ($15), have a single side-entry buckle that allows you to remove your baby from the side instead of from the top. Like most carriers we tested, the Snugli Comfort Vent by Evenflo ($20) has dual side-entry buckles so you can get your baby out from either side. That exit strategy may be a good idea if you want to move your sleeping baby to a crib without waking him.

Pacifier and toy loops and storage pockets. These are handy for the essentials.