Urolithiasis involving the upper urinary tract is a multifactorial disease that remains a significant health problem. A variety of intrinsic and extrinsic factors influence the incidence of disease in individuals and in all populations. At the level of the kidney, natural physicochemical processes result in crystalluria and the formation and growth of stones. Urinary supersaturation of some degree must be present but its significance may be altered by changes in urinary volume, pH, epitaxial relationships, and the presence or absence of naturally occurring inhibitors. A variety of environmental parameters acting through effects on the local urinary conditions determine which patients among a group of people inherently at risk will form stones. The above factors are considered herein with regard to the four major types of stone disease encountered today.