Kidney stones, or Renal calculi, are solid concretions (crystal aggregations) of dissolved minerals in urine; calculi typically form inside the kidneys or ureters. The terms nephrolithiasis and urolithiasis refer to the presence of calculi in the kidneys and urinary tract, respectively. Renal calculi can vary in size from as small as grains of sand to as large as grapefruit. Kidney stones typically leave the body by passage in the urine stream, and many stones are formed and passed without causing symptoms. If stones grow to sufficient size before passage -- on the order of at least 2-3 millimeters -- they can cause obstruction of the ureter. The resulting distention with urine can cause severe episodic pain, most commonly felt in the flank, lower abdomen and groin (a condition called renal colic). Renal colic can be associated with nausea and vomiting due to the embyrological association of the kidneys and the intestinal tract. Recurrence rates are estimated at about 10% per year.

Kidney stones are most commonly composed of calcium oxalate crystals, and factors that promote the precipitation of crystals in the urine are associated with the development of renal calculi. Conventional wisdom and common sense has long held that consumption of too much calcium can promote the development of kidney stones. However, current evidence suggests that the consumption of low-calcium diets is actually associated with a higher overall risk for the development of kidney stones. This is perhaps related to the role of calcium in binding ingested oxalate in the gastrointestinal tract. As the amount of calcium intake decreases, the amount of oxalate available for absorption into the bloodstream increases; this oxalate is then excreted in greater amounts into the urine by the kidneys. In the urine, oxalate is a very strong promoter of calcium oxalate precipitation, about 15 times stronger than calcium.

Other components of kidney stones include struvite (magnesium, ammonium and phosphate), uric acid, calcium phosphate, or cystine (found only in high urinary concentrations in people suffering from cystinuria). The formation of struvite stones is associated with the presence of urea-splitting bacteria, most commonly Proteus mirabilis (but also Klebsiella, Serratia, Providencia species). These organisms are capable of splitting urea into ammonia, decreasing the acidity of the urine and resulting in favorable conditions for the formation of struvite stones. The formation of calcium phosphate stones is associated with conditions such as hyperparathyroidism and renal tubular acidosis. The formation of uric acid stones is associated with conditions that cause high blood uric acid levels, such as gout, leukemias/lymphomas treated by chemotherapy (secondary gout from the death of leukemic cells), and acid/base metabolism disorders.

Renal calculi can occur due to other underlying conditions, such as renal tubular acidosis, Dent's disease and medullary sponge kidney, Many centers will screen for such disorders in patients with recurrent nephrolithiasis.