Correct determination of glomerular filtration rate (GFR) as an indicator of kidney function bears great importance because clinical decision-making depends on it at many occasions. During the last years, a huge body of literature has been published on estimation and measurement of GFR. The increasing pace with which novel estimation formulae, current and newer biomarkers, assay standardization procedures as well as invasive measurement methods of GFR were published, has been overwhelming for many clinicians. This concise review summarizes central issues in determining kidney function by listing some of the most important publications. It explains fundamental principles in GFR estimation and measurement, the influence of clinical characteristics on endogenous biomarkers, and obstacles in biomarker assessment. Thus, it is thought to be a guide for clinicians through the confusing jungle of GFR determination.