The impostor phenomenon (IP) is a psychological experience of intellectual and professional fraudulence despite objective markers of success. Research has focused on the prevalence and predictors of this phenomenon without establishing a solid conceptual understanding of the construct and its measurement. Furthermore, although IP is theorised to involve a mismatch between individuals’ expectations around performance and actual performance, few studies have tested the veracity of this claim with objectively quantifiable performance metrics. This thesis addressed these gaps with three broad aims: (1) systematically investigate and appraise the quality of common IP instruments; (2) streamline IP theory to increase conceptual clarity and (3) investigate relations between IP and wellbeing and academic achievement markers. To achieve this, a systematic literature review, several factor analyses and path models were conducted to address the research aims. The results of three studies involving a systematic literature review and empirical studies based on a total of 1845 undergraduate students are presented in this thesis. The findings of Study 1 demonstrated a gold standard instrument does not currently exist. Although IP instruments did not necessarily report poor psychometric properties, a notable absence of data against stringent quality assessment criteria resulted in lower ratings. Study 2 determined the factors that emerged from the IP instruments tended to reflect traditional theories of the construct with newer latent factors such as impression management. Studies 2 and 3 also identified individual differences in the IP and empirically supported the incongruence between self-assessments and actual performance. Furthermore, the well-established links to negative wellbeing outcomes were replicated. This thesis contributes to advancing conceptual understanding of the IP, its measurement and individual differences in real-world outcomes.