Cognition is a broad term that refers to the mechanisms by which we acquire, process, store and ultimately use information from the environment . It encompasses processes such as perception, learning, memory, and reasoning . The CLOSER British birth cohorts contain a wealth of information on cognition over the life course, and the cognitive measures available in these studies have been used to answer research questions in many different fields, e.g. education [2, 3], public health [4, 5], economics , psychiatry , psychology [8-10], and political science . However, these cognitive tests vary considerably both within and across the cohorts, and this has hindered studies of developmental trends and cross-cohort differences. Moreover, there is considerable heterogeneity in the quality and quantity of the documentation used to describe these cognitive assessments, and, to date, there has been no attempt to develop a uniform description of the key features of these instruments. Therefore, as a first step in facilitating developmental and cross-cohort studies, we provide a comprehensive description of the cognitive measures that are available in the five British birth cohorts. A companion report (available at https://www.closer.ac.uk/about-the-research-we-fund/data-harmonisation/) will assess the feasibility of harmonising the cognitive measures both within and across the cohorts.