This chapter seeks to familiarize the readers with acute kidney injury (AKI). AKI considers the disease as a spectrum of injury that extends from less severe forms to more advanced injury. To assess the severity of a current episode of AKI, risk, injury, failure, loss of kidney function, and end-stage kidney disease (RIFLE) criteria is used which is based on glomerular filtration rate (GFR) and UO and comprises of three levels of renal dysfunction and two clinical outcomes. Classification, diagnosis and investigations of AKI, fluid balance to correct electrolyte imbalance, renal replacement therapy (RRT), indications of RRT, etc. are discussed briefly throughout this chapter.