Testing of chemicals

Adverse Outcome Pathways, Molecular Screening and Toxicogenomics


The OECD Environmental, Health and Safety (EHS) Programme has been helping member countries to make better use of increased knowledge of how chemicals induce adverse effects in humans and wildlife, through the so-called Adverse Outcome Pathways.


What's new?

  • New AOP publications:
The OECD recently published three new publications on Adverse Outcome Pathways in iLibrary:
  • Cooperation with Scientific Journals:
  • In October 2021 and January 2022, two scientific journals, ‘Environmental Toxicology and Chemistry’ and ‘Environmental and Molecular Mutagenesis’ respectively signed a Memorandum of Understanding (MOU) with the OECD on co-operative activities in the field of Adverse Outcome Pathway (AOP) development, in order to increase the number of AOPs developed, reviewed and published. More information is available here.

  • On Tuesday 25 January 2022, the OECD held a webinar on Adverse Outcome Pathway (AOPs) co-operative activities between Scientific journals and the OECD. This webinar was organised primarily for Scientific Journal editors or publishers who are interested in reviewing/publishing AOPs and collaborating with the OECD in this activity. The objective was to present the basis for cooperation between scientific journals and the OECD and discuss the lessons learnt so far. The webinar replay and the presentations are available here

  • Publication of the Guidance Document for the Scientific review of AOPs:

The OECD AOP development, review and endorsement processes are explained in the Guidance Document for the scientific review of Adverse Outcome Pathways (AOPs), recently published. The document also provides guidance on the OECD quality standards required for the scientific review of an AOP developed on the AOP-Wiki platform. It defines the core principles associated with AOP scientific review in order to enable consistent scientific reviews to be conducted, regardless of who is doing the review, whether these be, for example a government agency, a scientific journal or the OECD. Following these guidance principles thus guarantees that an AOP has been reviewed in line with OECD standards and facilitates OECD endorsement of AOPs.


What is an Adverse Outcome Pathway?

The OECD launched a new programme on the development of Adverse Outcome Pathways (AOP) in 2012. An AOP is an analytical construct that describes a sequential chain of causally linked events at different levels of biological organisation that lead to an adverse health or ecotoxicological effect (see figure below). AOPs are the central element of a toxicological knowledge framework being built to support chemical risk assessment based on mechanistic reasoning.


Schematic representation of the AOP illustrated with reference to a number of pathways

AOP image


Watch our video: OECD work on Adverse Outcome Pathway


Publications on Adverse Outcome Pathways

Access all publications on Adverse Outcome Pathways. Find the OECD Series on Adverse Outcome Pathways on iLibrary  .


The AOP Development Programme

The AOP development programme is overseen by the Extended Advisory Group on Molecular Screening and Toxicogenomics (EAGMST). EAGMST members play an active role in the development of AOPs and can provide support to AOP authors during the development of an AOP.

Representation of the AOP Development Process at the OECD

‌ AOP development process


Cooperation on scientific review and AOP publication

The OECD is developing co-operation with Scientific Journals for the review and publication of AOPs.

This cooperation is formalised in the signature of a Memorandum of understanding (MOU). Scientific Journal Signatories take part in the scientific review of AOPs relevant to their field of expertise, with the view to achieve the following common objectives:

  • Increase the rate of AOP scientific reviews;
  • Offer the opportunity for journals to get more submissions for scientific articles;
  • Enable a double recognition through AOP publications in the scientific literature and at the OECD.

The conditions of the MOU are summarised here.

The following Journals have already signed a MOU with the OECD:


AOP Knowledge Base and Tools

The OECD's AOP Knowledge Base tools, constantly developed and refined, are a web-based platform which aims to bring together all knowledge on how chemicals can induce adverse effects, therefore providing a focal point for AOP development and dissemination.

Logo e AOP Portal


  • The e AOP Portal is the main entry point of the AOP Knowledge Base. The Portal enables search by key words in AOP titles and key events in the AOP Wiki. It houses the status of all AOPs in the OECD Work Plan and the official copy of OECD endorsed AOPs.

AOP Wiki logo

  • The AOP Wiki represents a joint effort between the European Commission - DG Joint Research Centre (JRC) and U.S Environmental Protection Agency (EPA). It represents the central repository for all AOPs developed as part of the OECD AOP Development Programme and provides a system that organises, via crowd-sourcing, the available knowledge and published research into a verbal description of individual pathways, using a user friendly Wiki interface. Information on AOP is collected in a qualitative and narrative way.



Missed our previous webinars on AOPs? Watch the video recordings

  • Adverse Outcome Pathways Framework - Tuesday 30 April 2019
    This first webinar on Adverse Outcome Pathways (AOPs) discussed the Adverse Outcome Pathway (AOP) framework. The AOP framework is a collaborative tool that applies an innovative approach for collecting mechanistic knowledge from various sources that can eventually support chemical safety assessment.

  • Adverse Outcome Pathways: Assembling and evaluating weight of evidence and quantitative understanding - Wednesday 15 January 2020
    This second webinar on Adverse Outcome Pathways (AOPs) focused on the importance of weight of evidence in the process of developing AOPs, the types and lines of evidences assembled, examples demonstrating the lines of evidence and understanding why quantitative AOPs are developed. The AOP framework is a collaborative tool that applies an innovative approach for collecting mechanistic knowledge from various sources that can eventually support chemical safety assessment.

  • Adverse Outcome Knowledge Base (AOP-KB) and AOP developing tips - Thursday 30 January 2020
    The third webinar on Adverse Outcome Pathways (AOPs) focused on the core information and telecommunication technologies applications that were built to support AOP development, management and dissemination. Tools and technical tips to facilitate scientific knowledge assembling and evaluations were also presented. The AOP framework implements a collaborative and innovative approach for collecting mechanistic knowledge from various sources that can eventually support chemical safety assessment.

  • Access all the presentations 


Find out more

Test Guidelines Programme

The OECD Test Guidelines Programme for the identification of new biomaker endpoints and in vitro test methods that are candidates to become part of OECD Test Guidelines.

Hazard Assessment Activities

The OECD Hazard Assessment activities for the development of Integrated Approaches to Testing and Assessment (IATA, using AOPs as a framework.
The OECD QSAR project for the identification of new methods/profilers for grouping chemicals.




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