<p>Research on cognitive effort has produced a vast scientific literature. In this paper, we propose a roadmap to describe key developments over the past 70 years. We highlight a shift from a research landscape dominated by questioning “What happens when we engage in effort?” to “Why we engage in effort?” The first question generates a research framework that we call the arousal framework, where effort is seen as a state of increased physiological activation. Research within this framework has flourished following the discovery of the ascending reticular activating system and has focused on the role of the locus coeruleus-noradrenargic pathway in modulating the intensity of cognitive control. The second question has been central to research driven by neuroeconomics and advances in neuroimaging and computational neuroscience, leading to what we call the valence framework. In this perspective, it is assumed that effort is inherently aversive, and that individuals engage in effort only if the benefits outweigh the costs. Within this framework, research seeks the neural correlates of expected value of cognitive control, with a spotlight on basal ganglia-prefrontal dopaminergic pathways. We describe exemplar work pertaining to each framework, highlighting their key epistemological features. We argue that future research on cognitive effort should aim to understand the complementarity and interactive nature of valence and arousal, integrating the two questions of “what happens” and “why.” We also argue that understanding effort requires looking more specifically into how individuals execute cognitively demanding tasks, i.e., which solution strategies they adopt.</p>