Renal sonography forms a basic part of routine diagnostic strategy. This textbook summarizes eighteen years of experience in diagnostic ultrasound. We want it to serve as a guide to both imagers and mere consumers of information. That is why we shall emphasize practical advice and diagnostic pitfalls; it is also why we shall often deal with the relations of sonography with other diagnostic procedures, which it may complement or replace, precede or follow, the purpose being to achieve efficiency at low cost. We shall limit our subject matter to the kidney itself and the neighboring retroperitoneal compartments, dealing only briefly with the lower urinary tract, which requires specialized procedures. We devoted considerable space to renal sonography in our book Clinical Atlas of Ultrasonic Radiography, published in 1973. Since then, nothing has changed and everything has changed. Nothing, because even then the differential diagnosis between a solid and a cystic mass, the etiologic diagnosis of a nonsecreting kidney, and the positive diagnosis of a traumatic juxtarenal hematoma were quite reliable, making possible drastic reductions in the indications for instrumental roentgenologic examinations. Everything, be­ cause improved resolution and grey scale imaging (already partially achieved in 1973, thanks to real time) have profoundly refined both anatomic and pathologic ultrasonic studies. And now high resolution real time imaging has revolutionized renal examination techniques, whereas Doppler is entering routine ultrasonic diagnosis.