The article presents an opportunity cost model of subjective effort and task performance, arguing that mental effort arises from the computation of opportunity costs associated with task execution. The authors propose that certain computational mechanisms, particularly those involved in executive function, can be used for only a limited number of simultaneous tasks, leading to opportunity costs. These costs are experienced as mental effort, which motivates the reallocation of computational resources to more valuable tasks. The model contrasts with explanations that attribute performance decrements to resource depletion or willpower. The authors review empirical findings from psychology and neuroscience, highlighting that performance reductions in tasks like vigilance tasks and self-control tasks are linked to the trade-offs between current and alternative tasks. They argue that the sensation of effort is a motivational signal that directs behavior toward more beneficial options. The model also explains how the perception of effort changes over time, as the relative value of alternative tasks shifts. The authors emphasize that the model accounts for both within-task and between-task performance decrements, and that the experience of effort is influenced by the expected utility of alternative actions. The model is distinguished from other accounts by its focus on opportunity costs rather than resource depletion or motivation. The authors suggest that the model can be tested empirically and that it provides a framework for understanding the dynamics of effort and performance in various contexts.The article presents an opportunity cost model of subjective effort and task performance, arguing that mental effort arises from the computation of opportunity costs associated with task execution. The authors propose that certain computational mechanisms, particularly those involved in executive function, can be used for only a limited number of simultaneous tasks, leading to opportunity costs. These costs are experienced as mental effort, which motivates the reallocation of computational resources to more valuable tasks. The model contrasts with explanations that attribute performance decrements to resource depletion or willpower. The authors review empirical findings from psychology and neuroscience, highlighting that performance reductions in tasks like vigilance tasks and self-control tasks are linked to the trade-offs between current and alternative tasks. They argue that the sensation of effort is a motivational signal that directs behavior toward more beneficial options. The model also explains how the perception of effort changes over time, as the relative value of alternative tasks shifts. The authors emphasize that the model accounts for both within-task and between-task performance decrements, and that the experience of effort is influenced by the expected utility of alternative actions. The model is distinguished from other accounts by its focus on opportunity costs rather than resource depletion or motivation. The authors suggest that the model can be tested empirically and that it provides a framework for understanding the dynamics of effort and performance in various contexts.