<p>Effort influences decisions to initiate and sustain physical and cognitive tasks. Although the perception of effort is central to human behaviour, its underlying mechanisms—especially in the cognitive domain—remain poorly understood. Building on knowledge from physical exertion, this article introduces the concepts of effort and effort perception through a multidisciplinary lens, integrating insights from exercise sciences, (neuro)physiology, and psychology. We begin by highlighting the inconsistent definitions of effort in the literature and propose a transdisciplinary definition: the intentional engagement of physical and cognitive resources to perform—or attempt to perform—a task. We then review methods for measuring effort, emphasizing the current limitations of physiological and performance-based variables. We argue that, when adequately contextualized as a unique perception dissociated from other exercise-related perceptions, the self-report of effort currently provides the most viable way to investigate effort. Next, we explore theoretical models explaining effort perception in physical tasks, focusing on the corollary discharge model as a promising theoretical framework. While this model offers valuable insights, it does not fully account for exerting effort during cognitive tasks. We suggest refining the corollary discharge model to encompass cognitive exertion, thus breaking the traditional silos between the physical and cognitive domains. Finally, we outline key challenges for future research: defining “resources” more clearly, developing reliable measurement tools for effort and its (neuro)physiological correlates, and determining whether effort perception is domain-general or domain-specific. We end by discussing the broad implications of our new account of effort for performance, health, and behavioural science.</p>