Microalbuminuria is a key measurement for detecting early renal disease, especially in diabetes. Importantly, microalbuminuria also has been shown to predict morbidity and mortality in the general population, where it may be associated with changes characteristic of the so-called metabolic syndrome: hypertension, elevated triglyceride levels, glucose intolerance, obesity, and low levels of high-density lipoprotein cholesterol. In several screening studies, microalbuminuria was found in 5% to 10% of patients with insulin-dependent diabetes mellitus (IDDM). The transition from normo-to microalbuminuria is seen in approximately 3% of patients with IDDM per year, and the risk increases with higher HbA1c, increases in blood pressure, and an above-normal albumin excretion rate. Parental hypertension is not seen more frequently in microalbuminuric individuals. To detect a correlation between albuminuria and blood pressure, 24-hour ambulatory blood pressure recordings represent a clear advantage. In patients with non-IDDM, microalbuminuria is associated with elevated blood pressure, and indeed an elevated prediabetic blood pressure may predict future renal damage, at least in some populations. The albumin molecule is stable in laboratory handling, and several assays are now available. However, there is considerable variability in excretion rate, but with repeated measurements, patients usually remain in the categories normomicro-, or macroproteinuric. Microalbuminuric patients with IDDM clearly show early glomerulopathy with abnormalities in the typical glomerular parameters, such as basement membrane thickening as well as mesangial expansion. Microalbuminuria is associated with a poor prognosis, and therefore, this parameter offers an ideal entrance criteria for clinical trials. Antihypertensive treatment clearly stabilizes or reduces microalbuminuria and seems to prevent a fall in glomerular filtration rate. Translated into clinical practice, this means that microalbuminuric patients should be treated with antihypertensive agents, in particular angiotensin-converting enzyme inhibitors (plus small doses of diuretics), unless microalbuminuria is very limited, stable, or decreasing. Monitoring for microalbuminuria is essential in monitoring complications among diabetic patients.