Focal segmental glomerulosclerosis (FSGS) is not a disease but a histological diagnosis on renal biopsy. It can be idiopathic or secondary to other entities, e. g. reflux nephropathy, HIV, obesity, hypertension, diabetes, etc. FSGS in children can be classified into primary, secondary and other types. FSGS is defined as involvement of some but not all glomeruli with sclerosis. Only a portion of the glomerular tuft is involved. There is glomerular capillary collapse with increased matrix. The earliest changes on light microscopy appear to be synechia between peripheral capillary loop and the glomerular tuft and Bowman’s capsule. This synechial region becomes sclerosed with type III collagen and increases in size while the tuft shrinks. Glomeruli not involved with sclerosis may show foot process fusion. Detection of even a single glomerulus involved with segmental sclerosis is sufficient to make the diagnosis.