This chapter reviews the origins of Attachment Theory, the Attachment Spectrum, and the Key Concepts of Attachment. These key concepts consist of the secure base, safe haven, proximity maintenance, and separation distress. This chapter includes detailed explanation of secure, insecure, avoidant, and disorganized attachment patterns and attachment seeking behaviors. Throughout the chapter, the reader is taught how to utilize and understand attachment theory as well as how to assess for the different styles of attachment patterns manifested by individuals and within the family system.