When it comes to the development of a substance use disorder (SUD) or other type of addiction, many factors come into play. It is well understood that there is a neurobiological component to the development of addiction. What role do other psychological factors play in the development of SUD or any other type of addictive behavior? Some studies suggest that both substance and behavioral addictive disorders are related to higher impulsivity, higher neuroticism, and lower conscientiousness. The comorbid and bidirectional relationship between mood and SUD, while well-documented, remains nuanced and challenging to diagnose when presenting together as mood and SUDs are risk factors for the development of each other, resembling, in many ways, a chicken-and-the-egg relationship. Substance use and anxiety disorders are also highly comorbid, and their relationship has been explored through multiple models. In this chapter, the first section will look at personality development and how personality can influence addiction. The following sections will examine personality and its interaction with mood and anxiety. Lastly, current clinical studies will be discussed that look at personality characteristics/disorders and SUD and/or addictive behaviors.