What's normal: It's not uncommon to gain 1-3 pounds after you lose a substantial amount of weight as normal levels of water and glycogen, a form of sugar (carbohydrates) stored in your muscles and liver, are restored. If you were on a low-carbohydrate diet, you'll likely gain back a little more, say 3-5 pounds, as you start adding carbs back into your diet.

What's not normal: Any additional weight beyond 3 pounds (or 5 pounds if you were on a low-carb diet) is most likely body fat, which, of course, you want to curtail. When to take action It's important to step on the scale once a week and to identify your "take-action" weight. For most people, this is 1-2 pounds above their target weight. When you exceed your take-action weight, go back to the habits that helped you succeed initially (provided they're healthful), such as cutting back on portions, drinking a meal-replacement shake or increasing your physical activity. It's critical to make a change quickly to get back on track.

James O. Hill, Ph.D., is director of the Center for Human Nutrition at Denver's University of Colorado Health Sciences Center and co-author of The Step Diet Book (Workman Publishing, 2004).