Factors identified in investigations of trait structure (e.g., the Big Five) are sometimes understood as explanations or sources of the covariation of distinct behavioral traits, as when extraversion is suggested to underlie the covariation of assertiveness and sociability. Here, we detail how trait covariation can alternatively be understood as arising from units common to functionalist and process frameworks, such as self-efficacies, expectancies, values, and goals. Specifically, the expected covariation between 2 behavioral traits should be increased when a specific process variable tends to indicate the functionality of both traits simultaneously. In 2 empirical illustrations, we identify a wide array of specific process variables associated with several Big Five-related behavioral traits simultaneously, and which are thus likely sources of their covariation. Many of these, such as positive interpersonal expectancies, self-regulatory skills, and preference for order, relate similarly to a broad range of trait perceptions in both studies, and across both self- and peer-reports. We also illustrate how this understanding of trait covariation provides a somewhat novel explanation of why some traits are uncorrelated. As we discuss, a functionalist understanding of trait covariation as arising through functionalist or process variables has implications for many basic issues in personality psychology, such as how personality traits should be measured, mechanisms for personality stability and change, and the nature of personality traits more generally.