Abstract Mode of Action (MOA) and Adverse Outcome Pathways (AOP) are conceptually similar constructs which organize mechanistic knowledge at a range of levels of biological organization. These constructs have been proposed as a basis to facilitate the integration and evaluation of mechanistic data for regulatory application. AOPs address chemically agnostic biological pathways from the molecular initiating event to adverse outcome. MOA analysis for specific chemicals includes consideration additionally of chemical space, metabolism and toxicokinetics. The AOP provides a convenient organizational construct as one element of broader strategies to advance more efficient and predictive integrated testing and assessment approaches. It is anticipated to increase focus on earlier key events (often measured in vitro or in non-test methods) at lower levels of biological organization by relating them to traditionally measured endpoints. Development of AOPs in an OECD program with formalized descriptions being assimilated in a publically accessible electronic knowledge base is anticipated to contribute to this advancement. These descriptions which include structured consideration of supporting qualitative weight of evidence and associated quantitation are anticipated to increase transparency in the extent of supporting data as a basis to consider potential regulatory application. A range of applications is envisaged, such as mode of action analysis and integrated assessment and testing strategies. Experience currently in application in hazard assessment for human health relates principally to priority setting for testing, due in part, to gaps in biological knowledge, lack of quantitation for dose response modelling and the need for additional development and acceptance of quantitative in vitro to in vivo extrapolation models.