Avian renal diseases are common in practice but are often undetected or misdiagnosed. Polyuria can be interpreted as diarrhea leading to inappropriate investigation and therapy. The avian urinary system differs from the mammalian. This article explains the anatomy and physiology of the avian kidney and focuses on the diagnosis of renal disorders. In particular, blood chemistry, urinalysis, radiography, urography, ultrasonography, computed tomography, and endoscopy (including biopsy) are explained and illustrated. Specific avian renal disorders and treatment possibilities are discussed.