Overeating, a poor diet, and lack of physical activity are by far the most important causes of obesity. Genetics may also play a role in the development of obesity.

Additional factors that contribute to obesity include:

• Pregnancy
• Lack of sleep
• Medical conditions:
• Cushing's disease
• Polycystic ovary disease
• Medications:
• Antidepressants
• Anticonvulsants
• Oral diabetes medications
• Antipsychotics
• Corticosteroids
• Beta blockers

Regardless of the cause, if the total calories (food energy) taken in is greater than the energy spent, then weight gain will occur.