Glomerular Filtration Rate (GFR) is recognized as the best global index of renal function, since its decline usually correlates with kidney functional mass. It is important to measure GFR to establish the risk of progression of CKD, calculate the dose of drugs excreted by the kidney, and prepare for invasive diagnostic or therapeutic procedures. Furthermore, renal function deterioration increases the risk of cardiovascular death in CKD and other non-communicable chronic diseases. GFR can be accurately measured using substances that completely filter trough the GBM and are not secreted or reabsorbed in renal tubules (inulin, iohexol, iotalamate, etc.); but these methods are usually reserved for research, given their complexity and cost. Creatinine clearance or mathematical formulas based on creatinine and bio-markers, such as age, sex, and race are used to estimate GFR. The most used ones are the Modification of Diet in Renal Disease (MDRD) and the Chronic Kidney Disease Epidemiology Collaboration (CKD-Epi). CKD is highly prevalent in the general population. General practitioners and clinicians should be aware of the reaches and limitations of the method chosen to evaluate GFR and able to recognize the circumstances in which glomerular hyperfiltration implies damage, and know how to treat to reduce progression to CKD.