In the fifteen years since the original volume of this series was written the problem of reflux has occasioned more controversy than any other topic in paediatric urology, and a full review of the subject is therefore required. Reflux undoubtedly justifies a full discussion by the frequency with which it occurs: its reported incidence varies considerably with the thoroughness of the investigation, but Scott and Stansfield’s (1968) finding of reflux in 55 per cent of children with recurrent urinary infection is not out of line with other reports. In the great majority of these children reflux is the only detectable abnormality of the urinary tract function, so that it has become customary to speak of primary reflux where there does not appear to be any bladder disorder, but, although a useful clinical classification, this term obscures the issue so often in doubt as to the part played by infection in the aetiology of reflux. Very seldom, in fact, can reflux be treated as a single disease entity; its presence is but one facet of a complex and variable situation and this chapter is therefore concerned with many aspects of urinary tract disorder which may be associated with reflux.