Hydrocele is a collection of fluid in the scrotum that results from a defect or irritation in the tunica vaginalis. It is usually painless and causes swelling of the scrotum. A hydrocele can be diagnosed based on physical exam findings like transillumination of the swollen area and ultrasound findings of a cystic mass around the testicle. While most hydroceles are benign, further investigation is needed to rule out other causes for scrotal swelling like hernia or testicular torsion that require urgent treatment.