With a view to assisting the evaluation of integrated approaches to testing and assessment (IATA) in regulatory decision-making within OECD Member Countries, this guidance document provides a set of principles for reporting defined approaches to testing and assessment that can be used as one of the components within IATA. Templates are also provided to enable a structured approach to their documentation in order to facilitate their evaluation. A defined approach to testing and assessment consists of a fixed data interpretation procedure (DIP) applied to data generated with a defined set of information sources to derive a result that can either be used on its own, or together with other information sources within an IATA, to satisfy a specific regulatory need. Thus, a defined approach to testing and assessment can be used to support the hazard identification, hazard characterisation and/or safety assessment of chemicals. This guidance is intended to be used alongside similar guidance aimed at harmonising the reporting of other IATA components such as QSARs, grouping and read-across strategies, and non-guideline in vitro methods.