The changing working environment and ageing workforce suggest that many workers will inevitably reach a career plateau, a stage where people regard future promotions as unlikely, or no longer feel challenged at work. Experiencing career plateau is found to be associated with negative job attitudes and performance in the workplace. However, existing studies have been mainly cross-sectional in design and few of them investigate the mechanisms and the conditions that influence the negative relationship. This study follows a quantitative longitudinal research design to further enhance the understanding of the relationships between career plateau and job outcomes. The job outcomes in this research include work attitudes such as job satisfaction, organisational commitment, turnover intentions and three aspects of job performance. Specifically, the three research aims of this thesis are: (1) to understand the causal directions between career plateau and job outcomes, (2) to examine whether employees’ unmet expectations may explain these relations and (3) to investigate whether employees’ age may modify the mediation effects of career plateau on job outcomes via unmet expectations. Three waves of data were collected from working individuals in various industries over an eight-month period. The majority of the participants were working in the UK or in Taiwan. Results showed that unmet expectations partially explain the negative relationship between career plateau and job attitudes over time but not for job performance. Furthermore, age did not affect the strength of the mediation effect of career plateau on the job outcomes through unmet expectations. The significance of this research is three-fold. First, it provides a new explanation for the unfavourable work attitudes of plateaued individuals. The findings that career plateaued employees have lower job satisfaction, lower organisational commitment and higher turnover intentions can partly be explained by unfulfilled expectations in receiving promotions or challenging tasks. Second, the study signals the danger of career plateau at any age, as younger and older workers were found to be equally influenced by career plateau. Finally, this is the first study in the area that seeks to resolve the debate over the directional relationship between career plateau and job outcomes. The implications in theory and in practice, limitations and suggestions for future research directions are discussed.