The protein adsorption layer (a.k.a. the "protein corona") that forms on the surface of colloidal nanoparticles plays an important role in their interaction with living matter. Thus, characterization of the protein corona is of utmost importance for understanding how exposure to nanoparticles affects the biological responses of cells and organisms. Although a lot of experimental studies have been reported in this direction, a comprehensive picture is still missing, in particular due to the multitude of different scenarios under which experiments have been performed. In this review an analysis of existing experimental data about the protein corona, and an outline for required future work will be given. In particular we review how existing simple analytical models such as the adopted Hill model may help to extract quantitative data from such experiments such as equilibrium dissociation and kinetic coefficients. Careful quantitative assessment of equilibrium and kinetic properties would allow for a comparison of protein binding data from the vast array of engineered nanoparticles, so that basic principles could be revealed. This review outlines that the field is in dire need of more quantitative studies to further our understanding of protein corona formation and its biological consequences.