Dancers have the body of the moment: lean, graceful, sculpted. So it's no wonder that the number of ballet-inspired barre classes, pioneered by dancer Lotte Berk back in the late 1950s, has grown exponentially in the last few years. Famous fans from Madonna to Zooey Deschanel credit the workout with firming up their middles and derrières without adding bulk.

"Barre focuses on the most challenging areas of the body—abs, butt, thighs—where all women want to see payoff," says Lashaun Dale, senior national group-fitness manager at Equinox, which has increased its number of barre classes by roughly 70% over the past two years due to demand. "Because the class is built around high repetition, you can achieve real results quickly." Here's what you will want to know before you go.

So is barre a form of ballet? Though it features pliés and other dance-school moves, barre is actually a blend of the classical dance form with yoga and Pilates. Each session targets your core and lower body as you make tiny pulsing movements, using your body weight as resistance. "You'll feel the muscle group get pretty shaky, but that means it's working," assures Alicia Weihl, director of training at Physique 57, in New York City. Your upper body will also get in on the sculpting action, thanks to warm-up planks and push-ups.

What about results? Not all bodies are created equal, of course, but you should start to see definition after about three weeks of twice-weekly classes, says Dale. Tools such as weighted balls help speed up progress. Bonus benefits: better posture and flexibility on account of frequent stretching, says Lis Settimi, co-owner of The Bar Method in Chicago.

Do I need to go to a special studio? No. Many boutique studios offer discounted classes to attract new clients, but then they hike rates to $30 to $40 per class. A better bet? Your health club. Several national chains, including Crunch and Equinox, let you belly up to the barre at no additional cost as part of their group-fitness classes.

Does a barre session count as cardio? Sorry—it's all about strength. If you're looking to get your heart pumping, you'll need to add 75 minutes a week of cardio (running, spin class). You will, however, burn fat, torching up to 345 calories an hour, says Jessica Matthews, MS, exercise physiologist for the American Council on Exercise. And your ankles won't throb, she adds: "Classes don't involve jumping, so they're joint-friendly."