The use of the calcineurin inhibitors cyclosporine and tacrolimus led to major advances in the field of transplantation, with excellent short-term outcome. However, the chronic nephrotoxicity of these drugs is the Achilles' heel of current immunosuppressive regimens. In this review, the authors summarize the clinical features and histologic appearance of both acute and chronic calcineurin inhibitor nephrotoxicity in renal and nonrenal transplantation, together with the pitfalls in its diagnosis. The authors also review the available literature on the physiologic and molecular mechanisms underlying acute and chronic calcineurin inhibitor nephrotoxicity, and demonstrate that its development is related to both reversible alterations and irreversible damage to all compartments of the kidneys, including glomeruli, arterioles, and tubulo-interstitium. The main question-whether nephrotoxicity is secondary to the actions of cyclosporine and tacrolimus on the calcineurin-NFAT pathway-remains largely unanswered. The authors critically review the current evidence relating systemic blood levels of cyclosporine and tacrolimus to calcineurin inhibitor nephrotoxicity, and summarize the data suggesting that local exposure to cyclosporine or tacrolimus could be more important than systemic exposure. Finally, other local susceptibility factors for calcineurin inhibitor nephrotoxicity are reviewed, including variability in P-glycoprotein and CYP3A4/5 expression or activity, older kidney age, salt depletion, the use of nonsteroidal anti-inflammatory drugs, and genetic polymorphisms in genes like TGF- and ACE. Better insight into the mechanisms underlying calcineurin inhibitor nephrotoxicity might pave the way toward more targeted therapy or prevention of calcineurin inhibitor nephrotoxicity.Clin J Am Soc Nephrol 4: 481-508, 2009. doi: 10.2215/CJN.04800908 T he introduction of the calcineurin inhibitor (CNI) cyclosporine in human kidney transplantation in the late 1970s revolutionized transplantation medicine, and made transplantation a preferable therapeutic intervention for end-stage renal diseases (1,2). In 1984, the potent immunosuppressive properties of another CNI, tacrolimus, were discovered, and tacrolimus was used successfully in human liver, kidney, and heart allograft recipients (3,4). Currently, 94% of kidney transplant recipients are discharged after transplantation with a CNI-based immunosuppressive regimen (5).The immunosuppressive properties of cyclosporine and tacrolimus result from inhibition of calcineurin, a calcium-and calmodulin-dependent phosphatase (protein phosphatase 3 [PPP3C], formerly PP2B). Intracellularly, these completely different molecules bind to cyclophylin and FKBP12 for cyclosporine and tacrolimus, respectively (6 -9). The competitive binding of cyclosporine-cyclophylin and tacrolimus-FKBP12 complexes to calcineurin inhibits phosphatase activity of calcineurin. This inhibition then suppresses the transcription of IL-2 via inhibition of the dephosphorylation and impaired translocation of the nucl...