In Software Engineering, personality traits have helped to better understand the human factor. In this chapter, the authors give an overview of important personality traits theories that have influenced Software Engineering and have been widely adopted. The theories considered are Myers-Briggs Type Indicator, Big Five Personality Traits, and Belbin Roles. The influence of personality traits has provided remarkable benefits to Software Engineering, especially in the making of teams. For software project managers, it is useful to know what set of soft skills correlates to a specific team role so as to analyze how personality traits have contributed to high performance and cohesive software engineering teams. The study of software engineers’ personality traits also helps to motivate team members. Creating teams that involve compatible individuals, each working on tasks that suit them, and having a motivated team improves team performance, productivity, and reduces project costs.