The use of knowledgeable informants is a particularly valuable tool for the diagnosis and assessment of personality disorder (PD). This review details the use of one particular type of informant—practicing clinicians—in PD research. We detail a wide variety of studies that have employed clinicians as an assessment source, including those focused on interrater agreement, comparative validity with other methods, cognitive factors of diagnosis, and opinion surveys. We demonstrate limitations, such as potential biases and limited convergent validity, which caution against the assumption that clinicians’ ratings should be considered a gold-standard. Nonetheless, we also highlight the potential value of research that focuses on clinicians due to its external validity to real-world practice settings. Finally, we outline several issues to consider when sampling clinicians, such as participation rate and sample size, and call for future research that collects ratings from clinicians using systematic, well-validated measures.