Accurate measurement of renal function is essential in patients with renal impairment (RI) so that drug dosages can be adjusted accordingly. Direct measure of Glomerular Filtration Rate (GFR) using plasma or urinary clearance is considered the best overall index of renal function. However, this is difficult to do in practice. Various creatinine-based equations for estimating GFR are discussed including eGFR, CKD-EPI and MDRD. Specific scenarios such as estimating renal function in patients at extremes of body weight, elderly patients or critically ill patients are also considered.