Significant progress has been made in the area of bacteriuria and pyuria in the last two decades. The development of quantitative bacteriological techniques helped define various types of bacteriuria. This made it possible to follow large numbers of bacteriuric patients and led to a clearer understanding of the natural history of bacteriuria. It is now clear that a significant proportion of healthy adult females have asymptomatic bacteriuria, which can follow a highly variable course in becoming persistent, in resolving spontaneously for good, or in relapsing frequently. We now know that the development of end-stage renal failure in such individuals (bacteriuric patients who have structurally and functionally normal urinary tracts) is extremely unusual. Therefore, interest has been justifiably switched to urinary tract infections in infancy and in childhood and to those infections that occur in association with renal scars and anatomical or functional abnormalities of the genitourinary tract. In this brief chapter, we will review some of these issues, but the limited scope will not permit exhaustive discussion of research literature or of controversial issues. The interested reader is directed to the appropriate articles as listed in the references.