Obesity refers to a body mass index (BMI) of 30 or higher (weight in kilograms divided by square of the height in meters). Obesity develops when an individual's energy intake is greater than energy expenditure, leading to excess weight gain. There are many variables that contribute to obesity, which is a disease that now affects approximately one-third of the United States' population.

Obesity affects individuals across broad demographics, but it is more prevalent in areas of low socioeconomic status in developed countries. Contributing environmental risk factors include the consumption of high-calorie foods, decreased physical activity, and poor sleep hygiene. There is likely a genetic predisposition to obesity, although in most cases there is not an obvious genetic cause.

Obesity predisposes to many chronic medical diseases including hypertension, hyperlipidemia, diabetes mellitus type 2, depression, hyperandrogenism, nonalcoholic fatty liver disease, pancreatitis, osteoarthritis, gout, and obstructive sleep apnea. Obesity also predisposes to cardiovascular diseases, which are among the leading causes of death in the United States. Obesity also increases the risk of malignancies including endometrial cancer, renal cancers, gallbladder cancer, esophageal adenocarcinomas, uterine cancer, liver cancer, colon cancer, female breast cancer, male breast cancer, ovarian cancer, gastric cancer, pancreatic carcinoma, and thyroid cancer, most of which are correlated with BMI. Obesity also increases the risk of more severe illness and complications for patients with COVID-19.

The following common skin conditions are seen more frequently in obese patients: hyperhidrosis and bromhidrosis, bacterial and candidal skin infections, onychomycosis, acne vulgaris, hirsutism, hidradenitis suppurativa, psoriasis, rosacea, androgenic alopecia, venous insufficiency, lipodermatosclerosis, and premature hair graying. Obesity may also be associated with rare skin conditions, including keratosis follicularis squamosa (Dohi), adiposis dolorosa (Dercum disease), cutis verticis gyrata, granular parakeratosis, and chronic obesity lymphedematous mucinosis.