As a fundamentally social species, humans rely on others to feel safe, secure, and make sense of the world around them. Consequently, people can feel disconnected from others in multiple ways. This chapter utilizes attachment theory as a framework for understanding varying ways in which people can feel alone. This chapter begins with an overview of attachment theory, which proposes that humans have a basic need to form affiliative bonds with others throughout their life, which function to provide felt security, emotional regulation, validation, and felt relief in response to a perceived threat. Breakdowns in attachment processes, such as anxious and avoidant reactions to unmet needs, can lead to greater propensity to experience varying forms of subjective disconnection when internalized. Specifically, this chapter reviews three types of subjective disconnection: emotional loneliness (perceived absence of a close or intimate relationship), social loneliness (perceived absence of an engaging broader social network), and existential isolation (perceived absence of shared perceptions and experiences). This chapter concludes with a review of relevant empirical research that underscores how attachment processes can inform experiences of subjective disconnection.