Epididymitis: This is often the most common cause of testicular swelling, especially in sexually active young men. It is an inflammation of the epididymis, usually caused by bacterial infection, which can lead to swelling, pain, and redness of the scrotum.

Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. It can present with testicular swelling, pain, and may be accompanied by epididymitis.
Hydrocele: An accumulation of fluid in the tunica vaginalis, a potential space around the testicle, leading to swelling of the scrotum. It can be primary (idiopathic) or secondary due to inflammation, trauma, or tumors.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. It can cause swelling and a feeling of heaviness in the scrotum, typically on the left side.

Testicular Torsion: A medical emergency where the spermatic cord becomes twisted, cutting off the blood supply to the testicle. It requires immediate surgical intervention to save the testicle. Symptoms include sudden, severe testicular pain, swelling, and elevation of the testicle.
Testicular Cancer: Although less common, testicular cancer can present with a painless testicular swelling. It is crucial to examine and investigate any testicular mass to rule out malignancy.

Spermatocele: A cystic structure that forms at the head of the epididymis, filled with sperm. It can cause swelling but is usually painless and discovered incidentally.
Cystic dysplasia of the testis: A rare congenital anomaly where the testis is replaced by multiple cysts, leading to testicular swelling.
Henoch-Schönlein Purpura (HSP): A systemic vasculitis that can rarely cause testicular swelling among its manifestations, which include purpura, joint pain, gastrointestinal symptoms, and renal involvement.