The primary purpose of this toolkit is to provide a resource for researchers from various disciplines interested in planning and evaluating programmes or interventions aimed at improving the health and development of infants and young children. The toolkit aims to provide an overview of issues affecting early development and its measurement; discuss the types of tests typically used with children under five years; provide guidelines for selecting and adapting tests for use in developing countries, and make recommendations for planning successful assessment strategies. The toolkit focuses on children who have not yet entered school, and are thus under six years old.