Adverse outcome pathway (AOP) is a conceptual framework that organizes existing knowledge concerning the causal linkage between a molecular-level perturbation of a biological system and the resulting adverse outcomes. AOP is currently gaining attention as a potential pragmatic tool in the fields of ecotoxicology and chemical risk assessment. The present paper provides an outline of the AOP framework and discusses the history and current status of studies related to AOP by analyzing available information from published literature and AOP knowledgebase (AOP-KB).