The purpose of this guidance is to harmonise the way non-guideline in vitro methods are described and thereby facilitate an assessment of the relevance of test methods for biological activities and responses of interest, and an assessment of the quality of data produced, irrespective of whether these tests are based on manual protocols or assay protocols adapted for use on automated platforms or high-throughput screening systems (HTS). This guidance outlines the elements considered relevant for providing a comprehensive description of an in vitro method to facilitate the interpretation of results and support scientifically defensible fit-for-purpose applications.