According to literature and to our experiences sonography should be the first investigation of the lower urinary tract in children, if there is suspicion of obstruction or malformation. In many cases the localisation of obstruction or the diagnosis of malformation can be found. Furthermore by sonographic follow-up studies of ureter obstructions by tumors, by concrements or after surgery, the normal radiographic investigations like urography or retrograde pyelography can be reduced. Sonography is a valuable enrichment for investigations of functional and morphological changes of the urinary bladder, especially for the classification of bladder tumors: whereas the diagnosis of urethral diseases still is found by clinical investigations and conventional radiological procedures.