Research pertaining to personality traits has largely focused on broad dimensions that define personality configurations, rather than on specific actions and behaviours that people engage in every day. While trait theorists hold the belief that individuals' personality traits predict their behaviours, there is no thorough conceptualization of behavioural manifestations of personality traits in the existing literature. The first goal of the present dissertation was therefore to conceptualize a model of behavioural manifestations of the Five Factor Model (FFM) traits. To achieve this goal, in the first article, a taxonomy of behavioural expressions of FFM traits was operationalized through the development of an instrument: the Behavioural Expressions of Traits Inventory (BETI). Results from Study 1 (N = 454) and Study 2 (N = 297) validated the proposed taxonomy by means of both exploratory and confirmatory factor analyses. The final version of the BETI comprised 30 items (6 items/subscale) that presented a clean factor structure. Concurrent validity results revealed that the taxonomy of behavioural expressions could be distinguished from FFM traits. The BETI also displayed good construct validity, satisfactory internal consistency values of all subscales, and no issues with social desirability. The second goal of this dissertation was to use this conceptualization of behavioural expressions of traits advantageously to examine an important fundamental topic: the processes at play in the associations between FFM personality traits and motivation through the lens of Self-Determination Theory (SDT). Results from emerging studies consistently revealed positive associations between beneficial FFM traits (openness to experience, conscientiousness, extraversion, and agreeableness) and autonomous motivation. Conversely, negative associations were obtained for neuroticism, a detrimental trait. The second article of this dissertation aimed to further our understanding of the associations between FFM traits and autonomous motivation by examining two potential mediators of this relationship: behavioural expressions of FFM traits and basic psychological need (BPN) satisfaction, a well-known antecedent of autonomous motivation. Two motivation domains central to the lives of undergraduate students were examined: academics and friendship. In Study 3 (N=635), undergraduate students completed online questionnaires. Structural equation modelling revealed a series of direct and indirect effects, as evaluated by Sobel's test of indirect effects: (1) FFM personality traits were positively associated with their corresponding behavioural expressions; (2) behavioural expressions of beneficial personality traits (openness to experience, conscientiousness, extraversion, and agreeableness) were positively associated with BPN satisfaction (autonomy, competence, and relatedness), while neuroticism was negatively associated with BPN satisfaction; (3) BPN satisfaction was positively associated with (a) autonomous academic motivation and (b) autonomous friendship motivation; (4) behavioural expressions of personality traits mediated the relationships between their corresponding trait and BPN satisfaction; (5) BPN satisfaction mediated the relationships between behavioural expressions of personality traits and (a) autonomous academic, and (b) friendship motivation and (6) behavioural expressions of personality traits and BPN satisfaction acted as sequential mediators of the associations between personality traits and (a) autonomous academic, and (b) friendship motivation. The sequential action of behavioural expressions of traits and BPN satisfaction as processes that explain the relationship between FFM personality dimensions and autonomous motivation is a novel idea that was put to a successful empirical test herein. Taken together, this research contributes to further our understanding of the intricacies involved in the joint study of the FFM model of personality traits and motivation as conceived by SDT.