Due to their high surface area to volume ratio, nanomaterials (NMs) have a high surface energy that they seek to lower by binding to available macromolecules from their surroundings, be they inert components of product formulations (e.g., polymers, oils, etc.), the proteins or lipids in living systems, or the natural organic matter components of water or soil. Thus, formation of a macromolecular corona around NMs is a ubiquitous phenomenon that can occur instantaneously upon contact with available macromolecules, whatever their source (e.g., product formulation, living organisms, or the environment). To date, and in the context of this present chapter, most work has focused on the interactions of NMs with proteins including blood serum or plasma (and the equivalent fetal calf or other animal sera for in vitro testing) and lung surfactant proteins, as intravenous and inhalation exposure routes are considered the most relevant for NMs, and sought to correlate the composition of the corona with NM uptake and impacts on living systems, as well as assessing impacts of binding on the protein conformation and functionality. In the environmental context, interactions of NMs with natural organic matter have focused primarily on the impacts of the interaction on particle stability and bioavailability, with a more limited amount of work on how the binding impacts NM interactions with living organisms. In this chapter, we review the principles and approaches to understanding the macromolecular corona and demonstrate that these are similar irrespective of the nature or source of the macromolecule, using the examples of proteins and humic acid to represent the human and environmental coronas, respectively. We suggest that in the environmental context, interactions of NMs with environmentally relevant macromolecules (e.g., exopolysaccharides in biofilms) may be a significant source of NM transformations altering their surface properties and subsequent biological interactions, and as such detailed understanding of the environmental corona and its effects are required as a matter of priority to enable predictions of NM-environmental behavior.