In this chapter, we introduce and discuss some of the most important and widely used models of personality. Focusing on trait theories, we first give a brief overview of the history of personality research and assessment. We then move on to discuss some of the most prominent trait models of the nineteenth century — including Allport’s trait theory, Cattell’s 16 Factor Model, Eysenck’s Giant Three, and the Myers–Briggs Type Indicator (MBTI) — before focusing on the Big Five Model (Five Factor Model), which is the most widely accepted trait model of our time. Next, we introduce alternatives to the Big Five that appear to be useful in the context of personalized services (the HEXACO and RIASEC models), and subsequently outline the relationships between all the models discussed in the chapter. Finally, we provide an outlook on innovative methods of predicting personality with the help of digital footprints.