This thesis presents the results of our attempt to unravel the, at the start of this project, still mainly theoretical concept of epistemic trust (ET). ET refers to the capacity to consider conveyed knowledge as trustworthy, relevant to the self, and generalizable to other contexts. Early negative childhood experiences may not only lead to attachment insecurity and impaired mentalizing but may also dispose an individual to adopt a rigid and pervasive hypervigilant position toward information coming from others, resulting in high levels of epistemic mistrust (EM). This disposition of EM is believed to increase the risk of developing psychopathology and might explain the profound rigidity and the ‘hard to reach’ character of patients with severe psychopathology. Although the concept of ET is essentially transdiagnostic, a more intrinsic relationship between epistemic mistrust and the development of personality disorders (PDs), more specifically Borderline PDs (BPD), is assumed. The general aim of this thesis was to clarify the concept of ET and to make ET assessable by developing and validating a clinically feasible measurement instrument to generate empirical support for the basic theoretical assumptions about epistemic trust and childhood adversity, attachment, mentalizing, and personality pathology.