The chapter explores how different forms of thinking about the future affect the self-regulation of energy mobilization for goal pursuit as assessed by its physiological underpinnings (i.e., cardiovascular response). Counter to what the popular self-help literature proposes, positive thinking can be detrimental to energy mobilization (i.e., energization) if it comes in the form of fantasies (free thoughts and images about the desired future) rather than beliefs (expectations). Indeed, fantasizing positively about a desired future leads to a decreased energization as indicated by a dampened cardiovascular response. Fantasy realization theory, however, specifies how positive fantasies can be used to wisely self-regulate energization for goal pursuit. According to the theory, the strategy of mental contrasting future and reality will increase or decrease energization, depending on a person’s high versus low expectations of success, respectively. Indeed, mental contrasting leads to an increased or decreased energization (measured by cardiovascular response and self-report) depending on expectations of success. The increased or decreased cardiovascular response in turn predicted performance in goal pursuit. Moreover, drawing on Hull’s conception of energization as an unspecific activation state, a recent series of studies found that energization triggered by mental contrasting in one domain may carry-over to another domain to fuel goal striving behavior with regard to an unrelated task. We discuss the relation of the findings to other models of physiological self-regulation, such as Brehm’s theory of motivation, the biopsychosocial model of arousal regulation, and excitation transfer theory. Finally, implications for interventions geared at improving self-regulation of effort are discussed.