Kidney stones (nephrolithiasis) form when dissolved minerals in urine become supersaturated and crystallize. The most common types are: Calcium oxalate (75-80%) — most common stone type overall, radiopaque on X-ray, associated with hypercalciuria, hyperoxaluria, and low urine volume. Calcium phosphate — often associated with renal tubular acidosis and hyperparathyroidism. Uric acid (5-10%) — the ONLY common stone type that is radiolucent (invisible on plain X-ray, visible on CT), forms in acidic urine (pH < 5.5), associated with gout and high-purine diets. Struvite/infection stones (5-15%) — composed of magnesium ammonium phosphate, form in alkaline urine infected with urease-producing bacteria (Proteus, Klebsiella), can form large 'staghorn' calculi filling the renal pelvis. Cystine stones (1-3%) — from inherited cystinuria, form hexagonal crystals in acidic urine. Stone prevention depends on type: increase fluid intake for all types (urine output > 2.5 L/day), dietary modifications, and type-specific interventions.