This PhD thesis examined the contribution of dual roles (i.e., work and study) to the career development of working students. It made a novel contribution to the literature by explicating the individual and joint contributions of demands and resources from dual role domains to their career development. Three studies addressed two aims: 1) to understand the relationships between work and study demands and resources and attitudinal (i.e., career optimism and perceived future employability) and behavioural indicators (career crafting) of career development in working students and 2) to examine the relative importance of the work versus study role and specific demands (challenge versus hindrance) and resources in these career development indicators. Cross-time and cross-sectional, as well as variable- and person-centred approaches, were used. Study 1 examined the relationships between work and study resources and working students' career optimism, both directly and via psychological needs satisfaction. Working students (N = 256; Mage = 20.08 years, 84% female) were surveyed twice (4-week lag). Time 1 (T1) study resources, but not work resources, predicted Time 2 (T2) career optimism. T1 study and work resources were related to T1 autonomy, competence, and relatedness satisfaction. However, only T1 autonomy satisfaction predicted T2 career optimism. The path from study resources to career optimism via autonomy satisfaction was significant, but only partially explained the study resources-optimism relationship. There was also a significant path from work resources to career optimism via autonomy satisfaction. Overall, 21% of the variance in career optimism was explained. Study 1 demonstrated that study resources enhanced career optimism, both directly and via autonomy satisfaction. However, work resources were only related indirectly via autonomy satisfaction. Therefore, resources from the study role are more important in career optimism. Study 2 extended on Study 1 by examining the relative contributions of challenge and hindrance demands from dual roles to another important attitudinal indicator of career development, perceived future employability (PFE). Moderation by the learning value of work (LVW), a specific work resource, was examined. Cross-sectional data were collected from 635 working students (Mage = 19.63 years; 70.4% female). Challenge work and study demands had positive relationships with PFE. Whereas hindrance study demands had a significant, negative relationship with PFE, hindrance work demands was not related. LVW had a positive relationship with PFE, and moderated the relationships between challenge and hindrance study demands and PFE. When challenge study demands were low, high LVW boosted PFE. When hindrance study demands were high, low LVW reduced PFE. LVW did not moderate the relationships between challenge and hindrance work demands and PFE. These findings underscore the importance of distinguishing between challenge and hindrance demands when examining how role demands affect career development. Study 2 added further evidence for the primacy of the study role in working students' career development. Study 3 departed from a variable-centred to a person-centred approach to identify distinct subpopulations of working students based on latent profiles of challenge and hindrance demands and resources in dual roles. It also examined profile differences on career optimism, PFE, and career crafting. Five demand-resource profiles were identified (Sample 1; n = 232) and replicated in an independent sample (Sample 2; n = 547). The most prevalent profile was characterised by average demands and resources across both roles (approximately one-third of participants). The least prevalent profiles were those characterised by high demands and low resources in a single role (<15%). Profiles characterised by low demands and sufficient resources in dual roles (approximately one-fifth) or high demands and low resources only at work reported higher career optimism, PFE, and career crafting. Profiles characterised by low study resources and high demands in dual roles (approximately one quarter) or only in the study role reported the poorest career development. Overall, this thesis demonstrated that experiencing dual role demands and resources is important in working students' career development, with study demands and resources being most relevant. It provided evidence that autonomy satisfaction explained relationships between role resources and career optimism, but further research is needed to fully explore these relationships. It also highlighted the differential roles of challenge and hindrance demands in career development and demonstrated the coexistence of dual role demands and resources within individuals. The findings can inform career development programs to support working students.