This review article is concerned with clinical and experimental data on glomerular number and size and includes a critical discussion of the relevant stereological methods. There is considerable evidence supporting the view that the size of the glomerular filter, in toto, is related to the inception and development of glomerular disease. Major divergences from the filter size are incompatible with normal structure and function. Furthermore, the hypothesis has been put forward that there exists a subset of individuals with minor divergences from the norm that is more susceptible to glomerular disease than the general population. Stereological methods for estimating glomerular number and mean glomerular size could be applied to set standard values. In addition, if the above hypothesis is confirmed, then they could identify on biopsy specimens the patients that belong to cohorts at risk of developing glomerular disease. Unfortunately, despite the development of new, robust methods for particle counting and size determination, such as the fractionator and the dissector, no universal standards of glomerular number and size have been agreed on to date. The major difficulties depend on 1) establishing a standard protocol of tissue acquisition and processing and 2) defining possible variations across age, sex, and ethnic subgroups. The considerable effort required to overcome these difficulties should be rewarded, however, by important advances in the understanding of the morphogenesis of glomerular disease and in the diagnostic and prognostic yield of renal biopsy.