This dissertation examines boredom as both a momentary state and a stable individual difference (boredom proneness). It addresses two questions: (1) Can persisting despite boredom during cognitive tasks be experienced as effortful? (2) Is boredom proneness associated with psychological and physiological indicators of stress? Across four studies, self-report, behavioral, and physiological measures were combined to capture subjective experience and underlying physiology. Studies 1 and 2 contribute to the debate on whether state boredom constitutes a meaningful source of demand in effort research. Participants completed an easy and a hard Stroop task, while repeatedly rating boredom-related effort and difficulty-related effort. Electrodermal activity was recorded in Study 1. Tonic pupil size and phasic pupil dilation were assessed in Study 2. Both studies examined whether effort during the primary task predicted performance in a subsequent task (physical in Study 1, cognitive in Study 2). Results indicate that boredom-related effort during easy tasks is linked to physiological correlates of effort and can carry over to impair subsequent cognitive task performance. These findings underscore the relevance of boredom-related effort for research on effort, including work on ego depletion and mental fatigue. Studies 3 and 4 propose, and partially test, a theoretical account of how boredom proneness may relate to stress, a topic that has received little attention. Study 3 focused on task-related psychological stress, whereas Study 4 assessed resting levels of psychological and physiological indicators of stress. Across studies, higher boredom proneness was consistently associated with higher stress levels. Overall, this dissertation highlights boredom-related effort as a relevant source of demand in effort research and positions boredom proneness as a potential vulnerability factor for elevated, possibly chronic, stress. Findings are discussed, and directions for future research are outlined.