Typically defined as a personality trait that is characterised by the setting of exceedingly high performance standards and overly critical self-evaluation tendencies, perfectionism has been associated with an increased risk for a variety of psychopathology, including anxiety disorders, depression, and eating disorders. However, due to the various ways in which the achievement-based construct has been defined and conceptualised in the literature, questions remain about its nature, structure, measurement, specific maintaining factors, and treatment. For example, it is unclear as to the extent to which dimensions of perfectionism are considered “functional” and “pathological” and more importantly, the nature of the relationship between the more adaptive facets of perfectionism and unfavourable performance situations (e.g., situations that imply a failure to meet performance expectations or standards). The present thesis had three broad aims. First, to investigate potential measurement issues with self-report instruments commonly used to assess multidimensional perfectionism. Second, the present research focused on broadening understanding of the cognitive, affective, and behavioural constructs that underlie both adaptive and maladaptive dimensions of perfectionism, and particularly how these are impacted during performance situations. Third, this thesis sought to gain a better understanding of the role of self-relevant constructs (i.e., self-concept and self-concept certainty) across adaptive and maladaptive dimensions of perfectionism. Findings from the present thesis contribute to a better understanding of the measurement issues associated with perfectionism measures and extend upon models of perfectionism by suggesting the role that a series of affective, cognitive (including the self-concept and self-concept certainty), and behavioural processes play in maintaining adaptive and maladaptive perfectionism