This chapter explores anuria and acute kidney injury (AKI) while applying physiological concepts in 3 patient cases: Patient A is a 22-year-old male presenting with hypovolemic shock following a motor vehicle collision; Patient B is a 33-year-old female with lupus presenting with headache and anuria; Patient C is a 70-year-old male who developed darker-colored urine. The pathophysiology of AKI can be categorized into prerenal, intrinsic renal, and postrenal classifications, each of which is characterized in these 3 patients. The chapter details the diagnostic process, including comprehensive history taking, physical examination, and essential laboratory tests such as serum creatinine, blood urea nitrogen, and electrolytes. Imaging studies are highlighted for ruling out postrenal causes. Management strategies focus on prompt restoration of hemodynamic stability to maintain a homeostatic volume balance and addressing the various complications that are common with AKIs. The chapter also addresses the importance of monitoring urine output and the use of renal replacement therapy if necessary. This case underscores the critical need for rapid diagnosis and treatment in preventing irreversible renal damage.