The term "microalbuminuria" qualifies an albumin excretion rate above the upper limit of normal individuals but below values giving positive results with conventional test strips. Its predictive value has been established for diabetic nephropathy in type I Diabetes, and for an excessive risk for mortality in type II Diabetes. Microalbuminuria is of glomerular origin in diabetes. It is secondary both to intra-renal haemodynamic changes and to altered permeability of the glomerular basement membrane. The various parameters involved in its genesis are reviewed.