Self-worth, the subjective value one ascribes to oneself as a person, is a central driving force of human action. The need for high self-esteem is axiomatic, and congruence between one's attitude towards oneself and one's actual abilities is a prerequisite for effectively coping with reality. However, despite amassing objective indicators of success, people often underestimate their abilities and achievements, which then stand in stark contrast to external performance expectations, which correspond to the actual successes achieved. This divergence between one’s own and social performance expectations by others can potentially lead to the feeling of being an Impostor. The impostor phenomenon describes the chronic feeling that one's achievements and successes are fake, no matter how many indicators of success have been amassed, as well as the fear of being exposed to social judgement. Growing scholarly interest in this phenomenon has led to increased research and new psychometric instruments to measure the interindividual characteristics of those affected. Common to all these instruments is measurement of an overall, expression despite the multidimensional conceptualization of the construct. In this dissertation project, we addressed this mismatch between the theoretical construct and psychometric operationalization by developing a multidimensional questionnaire for measuring the impostor phenomenon on a facet level. In our preliminary article, we developed and validated the six-scale Impostor-Profile. In a subsequent study, the first study of this dissertation project, we detected an Imposter-Profile total score by aggregating the six subscales. Further, we found that gender differences in Impostor expression are particularly evident at the subscale level. In the second study, in an experimental design we showed that high Impostor expression is related to external-unstable success attribution and internal-stable failure attribution. In the third and final study, we validated the English version of the Imposter-Profile and formulated and explored an explanatory model for the emergence and manifestation of Impostor feelings, which we termed the LHMIP. Our research thus focuses on expanding diagnostic possibilities in Impostor research, experimentally validating imposters’ non-self-serving attribution style, and contributing new impulses to etiological research through the exploratory model.