Many adults with significant forms of mental illness, including psychosis and personality disorders, experience deficits in metacognition that are reflected in a limited ability to describe and think about their own mental states and those of others. Above and beyond experiencing symptoms specific to their disorder, they may be unaware of their own emotions, unable to see their own thoughts as subjective and fallible, and struggle to form complex self-representations of themselves as unique beings in the world. Similarly, they have difficulties grasping the thoughts, feelings, and intentions underlying the behavior of others. Moreover, these patients may be relatively unable to use knowledge about themselves and others to respond to psychological and interpersonal challenges. In this introduction to this JCLP: In Session issue on metacognition and mentalizing, we detail different ways to understand these deficits and discuss three unique challenges these present to treating clinicians.