Most surfaces of engineered nanoparticles (NPs) are reactive toward biomolecules. Therefore, whenever NPs become immersed in biological fluids, proteins and other biomolecules bind to the NP surface, forming an adsorption layer (biomolecular corona) that modifies the NPs' physicochemical properties and subsequent interactions with living systems. Its exploration is a formidable endeavor owing to the enormous diversity of engineered NPs in terms of their physicochemical properties and the vast number of biomolecules available in biofluids that may bind to NPs with widely different strengths. Significant progress has been made in our understanding of the biomolecular corona, but even very basic issues are still controversially debated. In fact, there are divergent views of its microscopic structure and dynamics, even on physical properties, such as its thickness. As an example, there is no agreement on whether proteins form monolayers or multilayers upon adsorption. In our quantitative studies of NP-protein interactions by in situ fluorescence correlation spectroscopy (FCS) with highly defined model NPs and important serum proteins, we have universally observed protein monolayer formation around NPs under saturation or even oversaturation conditions. Here, we critically discuss biomolecular corona characterization using FCS and dynamic light scattering and identify challenges and future opportunities. Further careful, quantitative experiments are needed to elucidate the mechanisms of biomolecular corona formation and to characterize its structure. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.