An AOP starts with a molecular initiating event (MIE), usually followed by a series of sequential and causal key events (KEs) at cellular, tissue or organ levels of biological organisation and ends with adverse outcome(s) (AOs) defined as the disease or the pathological outcomes, which are targeted for regulatory decisions. These pathways allow systematic collection, organization, and simpler presentation of the information collected from in vitro, in vivo or in silico toxicological studies. As AOPs are not chemical-specific, they can be used in the identification of key toxicological data gaps and prioritization of research efforts and resources, thus supporting IATA . When biological interaction and exposure for fully characterized ENM-relevant stressors are identified, existing AOPs may thus facilitate regulatory-relevant RA.