Self-control failures are associated with many social problems including overweight, undue alcohol consumption, or aggression. It is therefore of vital interest to understand the processes that lead to self-control failures. The strength model of self-control (Baumeister, Vohs, & Tice, 2007) postulates that every act of self-control draws on a general, but limited resource that partly depletes with every use. The model is corroborated by abundant empirical evidence showing that after initial attempts at self-control, self-control failure becomes more likely, a phenomenon dubbed ego depletion. The strength model has received a lot of attention in scientific discourse and the media. However, it has also been heavily criticized. The recently postulated process model of self-control (Inzlicht & Schmeichel, 2012) denies the existence of a limited resource and assumes that a drop in motivation (invested effort) explains ego depletion effects. This model is also reconcilable with the bulk of the literature because both models predict similar behavioral patterns of results in many cases, although they are radically different on the theoretical level. The central assumption of reduced motivation after the initial exertion of self-control has been rarely tested. The existing evidence, which is based on self-reports, is weak. One reason for this may lie in the low validity of self-reports for inner states such as motivation. The goal of this research project is to investigate the role of motivational processes (mental effort) in ego depletion effects. To this end, we use established psychophysiological indicators of mental effort (pre-ejection period, systolic blood pressure) to avoid validity problems of self-reports. We put special emphasis on (a) experimental designs that allow testing conflicting hypotheses of both theoretical models, (b) high statistical power that allows interpreting statistically significant and non-significant findings, and (c) conceptual replications of central findings within the research project. By this means we make sure that upon completion of the project there will be a robust database to evaluate the role of mental effort in ego depletion effects.