In this entry, we define Intelligent Tutoring Systems (ITSs) and present a description of their core components. We outline a history of the development of ITSs with a focus on key issues that have driven change and innovation in ITSs from their inception to present day. We also present a brief case study on a specific ITS, Inq-ITS (Inquiry Intelligent Tutoring System; inqits.com), that is currently in use in 50 states in the US as well as many countries globally (12+) for the assessment of students' science competencies, and in turn, scaffolding students' science competencies. [This chapter was published in: "International Encyclopedia of Education, 4th ed., vol. 14," edited by R. J. Tierney et al., Elsevier, 2023, pp. 460-470.]