Assessment of renal function is usually considered to involve some measure of glomerular filtration; this contribution focuses on the methods by which glomerular filtration may be assessed with particular emphasis on those used in clinical practice. Such measures are of importance not only in helping to detect and monitor renal disease, but also in calculating dose adjustments in prescribing. However, it should be remembered that the kidney has many other roles, including tubular functions such as concentrating ability, hydrogen ion excretion and regulation of electrolytes, and the endocrine functions of α-hydroxylation of vitamin D and production of erythropoietin and renin. One-fifth of the cardiac output flows through the two kidneys, which weigh about 250 g (i.e. a flow rate of 1000-1200 ml/minute or 4 ml/g/minute). This blood flow is distributed among about 2 million glomeruli with a combined filtering area of about 1 m 2 . The rate of filtration across this membrane is governed by multiple factors including renal blood flow, intrarenal blood distribution, glomerular capillary pressure and glomerular capillary permeability. These in turn are partly controlled by afferent and efferent arteriolar tone, and by other intraglomerular factors such as mesangial cell tone. Normal glomerular filtration rate (GFR) is about 130 ml/minute (180 litre/day or 2 ml/second).