Weight Watchers is one of the oldest diet programs in America. Being practiced for more than 40 years, it works by the philosophy that the dieter should eat whatever he/she likes without sacrificing any of their beloved food or missing any of the meals. According to Weight Watchers, if we do not eat for long enough, any of the unused food is stored for emergency purposes and thus results in the accumulation of bodily fats.

The program works by controlling the number of protein intake of the dieter. Research has shown that protein helps to lose weight as varying the protein intake can affect the metabolism rate. The Weight Watchers program takes advantage of this relation and uses it to control the metabolism rate, which in turn controls the amount of calories that are burned by the body. Hence, having a good protein diet is actually conducive to weight loss as long as its intake is governed by Weight Watchers guidelines.

This program, thus, works by calculating food points based on a particular criteria. The dieter has to accumulate a certain number of target points within the given period.

The actual formula used by Weight Watchers is shown below:

Where:
p = Points
c = Calories
f = Fat Grams
r = Dietary fiber Grams
(Note: min{r, 4} equals the number of grams of dietary fiber or 4, whichever is smaller. In other words, only the first 4 grams of fiber "count.") (Consumer Research)
A Big Mac would have 14 Weight Watchers points if calculated by this formula.
Weight Watchers argues that by following its plan the dieter would sleep better, recover faster and have much more energy. It also imposes obligatory classes that every dieter must attend. These classes are there so that like-minded people can share their ideas and experiences about losing weight and can therefore guide the other members on these issues. These classes also act as an incentive to the newer members because they get to see many success stories and can gain from their experience in weight-loss. In the first class, the new member is weighed and then his/her ideal weight is calculated using the BMI (Body Mass Index); in the subsequent classes, the members are weighed at the beginning of the class to check on their progress and then they are allowed to share their experiences with the other members. The weights of these members are kept private, however. Once the member has achieved his/her ultimate weight goal, they have successfully completed the program and are given the status of the premium member. These members can then attend the meetings without paying any fees (to share their experiences with others and make sure they do not gain weight again) and are allowed to do so as long as they stay within a given weight range.

There are two main factors that differentiate Weight Watchers from many other diet plans. Firstly, Weight Watchers is inherently slow as it works by putting the dieter on a better, balanced diet rather than prescribing pills that would drastically reduce weight by other means. The actual amount of weight, thus, can vary greatly among different dieters but a loss of 2 pounds a week is the accepted maximum. Another aspect of Weight Watchers that is different from other diet plans is that there are no pills or special foods that must be consumed during this entire plan. (Healthy Weight Forum)
The program does not have a maintenance plan as such and it only has a weight loss phase that can be protracted. After the program is over, the dieter can still practice the same eating practices and maintain his/her weight or they can alternatively keep attending the free classes as premier members. The Weight Watchers program is a moderate-fat diet program, and therefore does not require any supplementation.
Exercising is not an essential part of this program, but is it is recommended to exercise 30 minutes a day, 5 days a week. The exercises give extra points to the dieter and they can be redeemed for food. This program does not put restrictions on any specific foods or prescribes any drugs, therefore there are no side effects of this program. However, limiting certain food categories can lead to deficiencies in certain nutrients especially calcium, iron and zinc. (Northwestern University) The cost of this program based on the assumption that the dieter looses 2 pounds a week would amount to roughly $150, though the cost may vary according to the country and the specific course (for example the online course).