Urolithiasis is the medical term for the presence of stones — calculi — in the urinary system: kidneys, ureters, bladder, or urethra. The stones form when urine becomes supersaturated with minerals, typically calcium oxalate, uric acid, or struvite, which crystallise and grow. Dehydration is the great risk factor, concentrating the urine; diet contributes through excess salt, animal protein, and oxalate-rich foods in susceptible people, while adequate calcium and citrate from fruit and vegetables protect. Passing a stone is famously painful. Prevention is largely dietary and behavioural: generous fluid intake above all, balanced diet, and treatment of underlying metabolic disorders. It is one of the most common urological conditions.