This course is designed to give an overview of psychometrics as a field. The lectures cover general topics that are relevant to many subfields of psychometrics: different types of modeling, the relation between psychometrics and neighboring fields, the history of psychometrics, and the function of psychometrics in society. During the course, PhD students interview leading psychometricians to learn how they view these central topics. It is important realize that What is Psychometrics? is *not* an introductory course on psychometric modeling, i.e., it does not introduce common psychometric models or concepts, and does not explain concrete psychometric data analysis techniques. Thus, this course is not suited for students who want to learn psychometric modeling practices.