Physicochemical changes to inorganic nanoparticles (NPs) in biological environments determine their impact. Blood, lymph, mucus, complete cell culture media and other biological fluids contain a large amount and variety of different molecules. NPs dispersed in these fluids are sensitive to such environments. One of the most significant alterations is the formation of the NP-protein corona (PC) as a result of the adsorption of proteins onto the inorganic surface. This process is currently gaining attention in the field of inorganic NPs since this spontaneous coating gives a biological identity to the composite NP-PC and determines the interactions between the NP and the host in living systems. Therefore, knowledge of NP-PC formation is crucial for understanding the evolution, biodistribution and reactivity of NPs inside organisms and, therefore, for the safe design of engineered NPs.