Studies of biological underpinnings of personality suggest that serotonergic functioning relates to certain personality traits. However, how to interpret the findings depends partly on assumptions about how personality is organized. These assumptions are reflected in the assessment devices used and also in how the data are examined. Review of evidence to date appears to link serotonin function to impulsivity and, to some extent, to hostility. The relation of serotonin function to anxiety proneness is far more questionable. Indeed, when such a relation occurs, it often takes a form opposite to the direction argued by theory. It is recommended that research use measures that discriminate adequately among personality qualities reflecting incentive sensitivity, threat sensitivity, and impulsiveness. Indeed, it is highly desirable to examine facets of each of these qualities separately.