The concept of a mode of toxicity is an understanding of the interaction of a chemical with cellular biochemistry from the level of exposure though to outcome, and therefore includes internal exposure, mechanisms of toxicity and disease pathways. With the exception of disease pathways these aspects are captured in this theme. This activity has a particular, but not exclusive focus, on early life effects because this is recognised as a susceptible stage of life of particular public health concern and addresses one of PHE’s seven Big Ambitions ‘Every Child ready to learn’.