Allergic Reaction: This is often the most common cause of swelling under the eyes, typically due to allergic reactions to cosmetics, pollen, or other environmental allergens. The swelling is usually accompanied by redness and itching.

Fluid Retention: Fluid retention, often due to hormonal changes, salt intake, or sleep position, can cause puffiness under the eyes.
Sinus Infection: Infections of the sinuses can lead to swelling under the eyes due to the close proximity of the sinuses to the orbital area.
Dermatitis or Eczema: Skin conditions like dermatitis or eczema can cause inflammation and swelling under the eyes.
Fatigue: Lack of sleep or fatigue can lead to puffiness and dark circles under the eyes.

Thyroid Eye Disease: Although less common, thyroid eye disease can cause swelling of the tissues around the eye, including under the eyes. It's crucial to identify this condition due to its potential to cause serious complications.
Cellulitis: A bacterial infection of the skin and tissues under the skin, cellulitis can cause swelling, redness, and warmth under the eyes and requires prompt antibiotic treatment.
Angioedema: A severe allergic reaction that can cause rapid swelling of the skin and mucous membranes, potentially leading to airway obstruction if not promptly treated.

Wegener's Granulomatosis: A rare autoimmune disorder that can cause inflammation of the blood vessels, including those around the eyes, leading to swelling.
Sarcoidosis: A condition characterized by the growth of granulomas (inflammatory cells) in various parts of the body, including the skin under the eyes, causing swelling.
Orbital Tumors: Although very rare, tumors in the orbit can cause swelling under the eyes, among other symptoms.