Analysis to determine the composition of a kidney stone is an important component to the assessment of a patient with kidney stones. Whether the patient has formed stones for the first time or recurrently, an effort should be made to obtain a stone for compositional analysis either by straining voided urine during a trial of stone passage or after shockwave lithotripsy (SWL), or by retrieving a stone fragment at the time of surgical stone treatment. Compositional stone analysis is typically performed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), or polarization microscopy.97 Although knowledge of stone composition may not reveal the etiology of stone formation in all cases, particularly for calcium-based stones, it can provide insight into etiology for some rarer stone types. The finding of struvite (magnesium ammonium phosphate) and/or carbonate apatite (a form of calcium phosphate) stones may be associated with the presence of chronic UTIs with urea-splitting bacteria. The finding of uric acid stones implies overly acidic urine and the potential for stone dissolution and prevention with alkali therapy. Furthermore, other medical conditions such as insulin resistance and type 2 diabetes mellitus, obesity, gout, and chronic diarrhea may be associated with uric acid stones and may require further investigation. Calcium phosphate (hydroxyapatite or brushite) stones may be associated with distal RTA as well as primary hyperparathyroidism and should prompt further workup for these conditions. Cystine stones are pathognomonic for cystinuria, a rare genetic disorder that impairs renal reabsorption of poorly soluble cystine, leading to stone formation. Lifelong monitoring and therapy are required for these patients.98 Stones composed of pure calcium oxalate monohydrate (rather than calcium oxalate dihydrate or mixed calcium oxalate monohydrate and dihydrate) may be associated with hyperoxaluria related to dietary intake, enteric hyperoxaluria, or primary hyperoxaluria type 1, an ultra-rare disorder of oxalate metabolism.99 Drug-induced nephrolithiasis represents approximately 1-2% of kidney stones, and compositional analysis can be diagnostic when the drug, or its metabolites, are identified.84 Repeating a compositional stone analysis on recurrent stones when a prior analysis has been performed can be useful. Variability in stone composition may occur due to sampling error, or changes in composition may occur over time due to medical and surgical interventions.100, 101