The Key Citations series was developed to provide a starting point for persons new to the various fields of impact assessment. The references provided are an indicative overview of the field and establish what might be regarded as the core literature. They include a selection of currently available textbooks published by commercial publishers, a selection of journal articles from the last 10 or so years, and key official documents. Some historically significant articles are also included. The means of determining key citations generally include consulting Scopus and Google Scholar and expert practitioners in the field. IAIA members contributing to this series acknowledge possible personal and regional bias and much difficulty in selecting only a few from among many excellent references in their fields.