Maladaptive beliefs are proposed to be critical in the development and maintenance of anxiety-related disorders. One commonly reported maladaptive belief domain concerns negative beliefs people hold about losing control. These concerns about the likelihood and consequences of losing control over one’s behaviour, thoughts, emotions and/or physical reactions appear to be prevalent in clinical and non-clinical samples. This program of research aimed to better assess, define and delineate the nature of these beliefs using a multi-method approach. In Study 1, undergraduate psychology students (N = 126) were given false feedback that they were either at high or low risk of losing control, and then completed a social interaction task with an actor. The belief that one was at high risk of losing control led to greater anticipatory anxiety leading up to the social interaction task, significant doubt about one’s social performance and significantly more negative post-event processing. In Study 2, an unselected sample of undergraduate participants (N = 21), half of whom met criteria for one or more anxiety-related disorders, was interviewed about their beliefs about losing control. Losses were defined as negative, multifaceted cognitive-behavioural phenomena which included thoughts, behaviours and emotions. Common consequences were perceived harm to oneself or others, powerlessness, and unpleasant emotions during (e.g., sadness, frustration, anxiety) and following (e.g., regret, shame, humiliation) a reported loss of control. In Study 3, undergraduate students (N = 440) completed an expanded set of items designed to better measure maladaptive beliefs about losing control, leading to the assessment of the Beliefs About Losing Control Inventory, Second Edition (BALCI-II). An exploratory factor analysis indicated the BALCI-II captured several domains of feared consequences of losing control: 1) overwhelming emotions 2) dangerous behaviour and 3) madness and 4) inflated beliefs about probability/severity of those losses. The BALCI-II was found to be psychometrically sound and predictive of symptoms of OCD and SAD, above and beyond existing disorder-specific maladaptive beliefs. Implications for cognitive theory and therapy are discussed.