Acute renal failure accompanied by an increase in retention parameters and oligo-anuria is a common medical problem. The most important medical task in nephrology is a fast diagnostic differentiation between prerenal-, renal- and postrenal failure. Using simple techniques such as case history, medical examination, ultrasound and specific analysis of serum and urine, it is possible to obtain a relatively certain diagnosis. Radiographic techniques with radiocontrast agents are contraindicated because of nephrotoxic side effects. The treatment of acute renal failure is based on the underlying disease. Apart from symptomatic treatment, there are only very few medications available to improve the excretory kidney function and diuresis. Complications like hyperkalemia, hyperhydration, metabolic acidosis and uremia require early renal replacement therapy. Especially in cases of extreme hyperkalemia, an immediate beginning of therapy is important. In the medical practice, emphasis should be placed upon the prevention of renal failure.