Acute kidney injury and chronic kidney disease are both commonly observed in cancer patients. Moreover, many anti-cancer drugs are eliminated from the body through the kidney. Therefore it is important to quantify kidney function in every cancer patient. However, there is a lot uncertainty regarding the best method to quantify kidney function in cancer patients especially so in the context of drug dosing. In this presentation I will discuss the different methods to measure and estimated kidney function and to guide drug dosing. Recent studies have shown that the Cockcroft- Gault formula to calculate creatinine clearance is inferior to newer formula to estimate kidney function in both the general and cancer population. Also to guide drug dosage, cumulative evidence demonstrates that the Cockcroft- Gault formula should not be used anymore and probably the eGFR CKD-EPI formula is the preferred formula.