Evaluation of the histological changes found in kidney biopsies taken during the investigation of children , with renal disease is complicated by the fact that in many instances the abnormalities seen by light microscopy are slight or absent, and may be prone to subjective errors of interpretation. In this thesis a number of renal biopsy specimens have been studied from 106 children presenting with a nephrotic syndrome, with idiopathic recurrent haematuria or with evidence. of renal involvement in anaphylactoid purpura. In addition to a morphological classification, more objective histological techniques have been applied in analysing the changes found. Semi-quantitative methods of assessing the over-all degree of glomerular and tubular damage present have been used, and in addition, differential counts of the various types of cell composing the renal glomeruli have been made. These counts were found to be particularly, valuable in the interpretation of slight degrees of 'proliferative' change in the renal glomeruli, the presence or absence of which is often difficult to establish subjectively. A number of preliminary studies were made to establish the reproducibility of the techniques of histological assessment used, and the renal biopsy findings were correlated with various clinical data. In children presenting with a nephrotic syndrome the histological findings were correlated both with the response to treatment, and where appropriate, with the clinical outcome. Amongst the children with nephritis following anaphylactoid purpura, a very good correlation was found between the degree of ✓structural damage revealed on renal biopsy and the subsequent clinical course. In these biopsy specimens, and also in those taken from children presenting with idiopathic recurrent haematuria in which proliferative changes were found in the renal glomeruli, differential glomerular cell counts showed that in the majority of cases proliferative changes were diffuse rather than focal in distribution, affecting all the glomeruli present. In patients with idiopathic recurrent haematuria, renal biopsies were histologically normal in many instances. A very close correlation was found between the amount of protein excreted in the urine by these children and the presence of abnormalities on renal biopsy.