Heart failure
Is a predominant cause, where inability of the heart to pump effectively leads to congestion of blood in the veins returning the blood to the heart. Subsequently, fluid accumulates in the lower extremities.

Chronic Venous Disease 
Chronic veins problems in the legs stemming from leaky valves (Varicose Veins) or obstruction in the veins (Deep Vein Thrombosis), results in increased venous pressure and oedema leading to swollen legs typically in one leg and. If caused by obstruction in the veins, the leg swelling may also be accompanied by pain and redness, warranting prompt diagnosis and treatment.

Kidney & Liver disease 
Kidney disease plays a significant role in leg swelling. In kidney failure the body can lose significant amounts of protein which leads to inability to retain water in blood stream and its accumulation in the tissues under the skin, leading to swollen legs. Liver diseases can also produce the swollen legs through the same mechanism.

Lymphatic Problems 
Lymphatic obstruction (Lymphedema), causes protein-rich fluid accumulation of lymph fluid under the skin leading to swollen legs.

Inflammation and Infection 
Inflammatory conditions such as Arthritis, or skin infection such as Cellulitis can cause localised leg swelling.

Medications 

Certain drugs, such as calcium channel blockers (Amlodipine, Nifedipine) corticosteroids, and nonsteroidal anti-inflammatory drugs (NSAIDs), are known to induce leg swelling (oedema).