If you're already in shape:

Kickboxing is a great calorie burner. Be sure the instructors know about martial arts as well as aerobics; there are a lot of moves that if done incorrectly can be harmful to knees, shoulders and elbows.

Spinning (indoor cycling set to music) offers an intense workout requiring stamina and a good imagination!

Pilates marries calisthenics with stretching movements and requires good muscle control.
 

If you're on the mend:

Weight lifting allows you to focus on healthy parts of the body while you're nursing an ankle or knee injury.

Stationary biking can actually strengthen bad knees.

Rowing is great exercise for the upper body while your bad hip (knee, ankle) mends.

Swimming and water aerobics help repair a sore back and are also good cardio exercises.
 

If you're overweight:

Your first goal should be to start exercising, even if it's just 5 minutes a day.

Yoga and low or non-impact aerobics are a good combination, which explains why most aerobics classes end with 10-15 minutes of "cool down" stretching.

Water aerobics and swimming offer cardiovascular activity using water as moderate resistance.
 

If you're over 65:

Weight training jump-starts the body with terrific results, even for people in their eighties. Twice-weekly workouts of 30 minutes each have been shown to reduce the risk of osteoporosis and fractures among postmenopausal women.

Tai Chi may effectively treat fibromyalgia and arthritis. It also lowers blood pressure almost as much as 30 minutes of daily brisk walking.

Ai Chi is a water program that coordinates breathing techniques with flexibility, relaxation and strength.

Most styles of yoga involve gentle stretching exercises, easing pain from arthritis while increasing flexibility and balance.
 

For the kids:

Exercise helps strengthen growing bones, so encourage children to be active for at least 30 minutes a day.

Contrary to myth, children can benefit from weight training, as long as it's done under adult supervision and with a maximum of 25 pound dumbbells.

Most clubs won't allow kids under 15 to use their facilities, so it's best to set up a home gym.
 

For Everyone, some rules apply regardless of age or physical condition:

Talk to a fitness professional to tailor a program that's right for you (see Trainer Tips).

Start conservatively, doing slightly less than you think you can.

Monitor yourself during (and up to 24 hours after) a new activity before proceeding to the next level.

Consult your physician before beginning an exercise regime.