<p>Knowledge of glomerular filtration rate (GFR) is essential for the treatment and monitoring of children with a malignancy. Ideally, GFR is measured using the gold standard technique of renal inulin clearance. Still, this procedure is too invasive and time-consuming for routine clinical practice. Also, the armamentarium of markers suitable for GFR measurement (mGFR) has changed in the last decades, with iohexol clearance now being used in many countries, both in Europe and the US. For daily monitoring of kidney function, cystatin C has proven to be an important adjunct to serum creatinine, the standard marker of GFR. This review gives a comprehensive update on the indications and limitations of mGFR methods as well as the estimation of GFR from serum markers (eGFR) with special emphasis on children treated for a malignancy.</p> Graphical abstract <p></p>