Excessive reassurance-seeking (ERS) is a common problem among individuals dealing with emotional and/or psychological difficulties. Prior research on ERS has focused almost exclusively on the potential consequences of this behaviour in the contexts of Depression and Hypochondriasis, and this research has shown that ERS contributes to interpersonal difficulties and emotional distress. Despite anecdotal evidence that ERS is a hallmark feature of Obsessive-Compulsive Disorder (OCD), comparatively few studies have examined OCD-related ERS. The goal of the present research was to examine various cognitive, behavioural and affective processes that may be involved in the perpetuation of ERS, specifically within the contexts of OCD and Depression. Given the purported functional equivalence between OCD-related reassurance seeking and compulsive checking (Rachman, 2002), the current investigations also aimed to compare ERS and repeated checking activity across a number of important domains (i.e., content, precipitating factors, function and termination criteria). Toward these goals, Study I employed a semi-structured interview with clinical (OCD and Depression) and non-clinical individuals to examine factors involved in the onset, maintenance and termination of ERS and repeated checking. Results revealed that individuals with OCD tend to seek reassurance about perceived general threats (e.g., fire, theft), whereas ERS tends to be focused on perceived social threats (e.g., abandonment, loss of support) among depressed individuals. Clinical participants reported greater anxiety, sadness and perceived threat in association with ERS and repeated checking than healthy control participants. Study 2 examined how manipulations of threat, responsibility, and ambiguity of feedback impacted upon non-clinical participants' anxiety and compulsive urges (to seek reassurance and to check) in a series of experimental vignettes. Consistent with hypotheses, higher levels of perceived threat, responsibility and ambiguity of feedback were associated with greater anxiety and compulsive urges. Results also suggested that perceived threat and responsibility partially mediated the effects of ambiguity of feedback on anxiety, urges to check, and (for threat) urges to seek reassurance. The collective results of these studies are discussed in terms of cognitive and behavioural models of OCD, and directions for future research are suggested