In the ICD-11 diagnostic guidelines, preoccupation has been introduced as the new core symptom of adjustment disorder. Despite this essential innovation, preoccupation has so far largely been defined as rumination and worry and does not feature a distinct character as an independent symptom. In order to investigate the nature of preoccupation, various cognitive approaches are evaluated and linked to preoccupation. Furthermore, the aim of this review is to define preoccupation more precisely and to distinguish it from other symptoms in psychopathology. The evaluation of key features of cognitive dissonance theory, attention bias theory, memory theories, and other cognitive paradigms indicates that preoccupation is constituted by a complex interaction of cognitive-emotional mechanisms. In addition, this review implies that preoccupation in AjD can be defined as stressor-related factual thinking, which is time-consuming and often associated with negative emotions. It is assumed that rumination and dysfunctional worry serve as reactive processes to cope with preoccupation. For further distinction, this review presents similarities and differences of preoccupation and other symptoms, including negative automatic thoughts, flashbacks, and yearning. Finally, implications and suggestions for future research on preoccupation are offered. Overall, it is plausible that preoccupation is not only associated with adjustment disorder but also possesses a transdiagnostic character.