The cognitive reactivity of perfectionists plays a key role in perfectionism as a diathesis for distress and health problems. This chapter provides an overview of the influential history of perfectionism from a cognitive perspective. It illustrates the relevance of the cognitive elements of perfectionism by describing research on individual differences in perfectionistic cognitions. The chapter expresses that perfectionistic cognitions are an important supplement not only in clinical assessments, but also in perfectionism research. The cognitive elements of perfectionism are also reflected in the frost multidimensional perfectionism scale (FMPS). At a deep structural level, perfectionists will have multiple self-representations that can become activated depending on current life experiences. Cognitive perfectionism at the propositional level should also reflect the approach–avoidance conflict that characterizes many perfectionists. Cognitive activation can include various forms of uncontrollable worry and cognitive products such as mistake ruminations and stress-related rumination.