In this chapter, we examine the tendency for vulnerable perfectionists to obsessively ruminate and cognitively perseverate in response to stress and to feelings of distress, and we introduce a cognitive theory of perfectionism. Our analysis includes an overview of the studies showing that various elements of trait perfectionism are associated with worry and ruminative brooding in maladaptive ways, which have significant implications for the onset and persistence of health and mental health problems. The vulnerability of both self-oriented perfectionism (SOP) and socially prescribed perfectionism (SPP) is jointly emphasized, as it is shown that both SOP and SPP are linked consistently with worry and rumination. The costs and consequences of this excessive cognitive perseveration are outlined within the context of a conceptual framework for our proposed cognitive theory of perfectionism. A central premise of perfectionism cognition theory is that certain perfectionists are chronically engaged in overthinking. We propose an expanded conceptualization of rumination in perfectionism that includes the tendency for perfectionists to experience frequent automatic thoughts about their need to be perfect and to engage in excessive mistake rumination, failure perseveration, and social comparison rumination. Our analysis of “perseverating perfectionists” emphasizes that their propensity to engage in various forms of ruminative thinking is deeply rooted in self and identity issues involving chronic self-uncertainty, self-doubt, a need for self-validation, and a chronic self-focus on the acceptability of dispositional characteristics. The role of cognitive perseveration in the health and mental health problems of vulnerable perfectionists is discussed.