Urinary tract stone formation is a multifaceted process. Urinary tract stone crystalline components are of six types: calcium oxalate, calcium phosphate, bacterial related, purines, or cystine. The majority of urinary stones are admixtures of two or more components, with the primary admixture being calcium oxalate with apatite. The formation of urinary tract stones is a result of increases in urinary supersaturation and the subsequent formation of crystalline materials. The mechanism of formation of crystalline particles in the urine is based on the thermodynamic state of the urine chemistry. The natural progression of the urine chemistry leading to stone development is urine saturation, urine supersaturation, crystal nucleation, aggregation, the retention of crystals by the urothelium, and the continued growth of the stone on the retained crystals. When the concentration of the salt components increases beyond the saturation level, a state of supersaturation exists in the urine, which is thermodynamically unstable. Supersaturation leading to nucleation is controlled by the thermodynamic free energy of the solution. The process of nucleation results in a reduction of excess free energy to a more thermodynamically stable environment. Aggregation appears to be the major mechanism for crystal growth. A final factor that is important in the effective growth of renal calculi is the retention of microcrystals in the urinary tract, possibly correlated with prior injury.