This introductory essay to this special issue demonstrates that action research has a vital role in evidence-informed practice in academic libraries. This special issue of "College and Research Libraries" ("C&RL") proudly features a selection of action research studies by participants of the Association of College and Research Libraries (ACRL) program "Assessment in Action: Academic Libraries and Student Success" (AiA). Just over 200 institutions have participated in the AiA program, with a librarian from each leading a campus team in developing and implementing an action learning project that examines the impact of the library on student success and contributes to assessment activities on campus. As is typical of research approaches that are becoming part of the academic mainstream, confusion and debate exist about what action research is and to what standards it should be held. This special issue aims to help "C&RL" readers learn more about action research as an approach to scholarship and showcase examples of fruitful action research studies undertaken by AiA teams.