Obesity traditionally has been defined as a weight at least 20% above the weight corresponding to the lowest death rate for individuals of a specific height, gender, and age (ideal weight). Twenty to forty percent over ideal weight is considered mildly obese; 40-100% over ideal weight is considered moderately obese; and 100% over ideal weight is considered severely, or morbidly, obese. More recent guidelines for obesity use a measurement called BMI (body mass index) which is the individual's weight multiplied by 703 and then divided by twice the height in inches. BMI of 25.9-29 is considered overweight; BMI over 30 is considered obese. Measurements and comparisons of waist and hip circumference can also provide some information regarding risk factors associated with weight. The higher the ratio, the greater the chance for weight-associated complications. Calipers can be used to measure skin-fold thickness to determine whether tissue is muscle (lean) or adipose tissue (fat).
Much concern has been generated about the increasing incidence of obesity among Americans. Some studies have noted an increase from 12% to 18% occurring between 1991 and 1998. Other studies have actually estimated that a full 50% of all Americans are overweight. The World Health Organization terms obesity a worldwide epidemic, and the diseases which can occur due to obesity are becoming increasingly prevalent.
Excessive weight can result in many serious, potentially life-threatening health problems, including hypertension, Type II diabetes mellitus (non-insulin dependent diabetes), increased risk for coronary disease, increased unexplained heart attack, hyperlipidemia, infertility, and a higher prevalence of colon, prostate, endometrial, and, possibly, breast cancer. Approximately 300,000 deaths a year are attributed to obesity, prompting leaders in public health, such as former Surgeon General C. Everett Koop, M.D., to label obesity "the second leading cause of preventable deaths in the United States."

The primary warning sign of obesity is an above-average body weight.
If you are obese, you may also experience:
Trouble sleeping
Sleep apnea. This is a condition in which breathing is irregular and periodically stops during sleep.
Shortness of breath
Varicose veins
Skin problems caused by moisture that accumulates in the folds of your skin
Gallstones
Osteoarthritis in weight-bearing joints, especially the knees
Obesity increases your risk for:
High blood pressure,
High levels of blood sugar (diabetes)
High cholesterol
High triglycerides levels

Consumption of high calorific foods like saturated and trans fats and sugars.
Addiction to television and other hand-held devices that cause a sedentary lifestyle.
Mid-life weight gain; Older people are at a greater risk of weight gain compared to younger individuals; especially older women who are in their menopausal age.
Illnesses like hypothyroidism (underactive thyroid), Cushing's syndrome (increase in the levels of the stress hormone, cortisol) and Polycystic ovarian syndrome (PCOS) (occurring in 5-10% of women of child-bearing age) have been linked to obesity.
Usage of Drugs like steroids, oral contraceptives, antidepressants, antiepileptics, antihypertensives and insulin is commonly followed by weight gain.  
Heredity: Obesity tends to run in families. A clearer role of genetics helps in prevention of obesity for those who are most vulnerable. Eighty percent of the offspring of two obese parents become obese.

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