Loneliness has been addressed theoretically for many years from psychological and social science perspectives and, more recently, by public health. However, not much is known about the complementary nature of the different existing theoretical frameworks. The aim of this chapter is to chart the main theories of loneliness relevant to older adults within micro-, meso-, and macro-level perspectives and to describe how the micro-, meso-, and macro-levels complement one another. At the micro-level, we present developmental, cognitive, existential, personality, and biological theories of loneliness as they apply to later life. Moving to the meso-level, we summarize frameworks on interpersonal networks and relationships which play a crucial role in shaping the experiences of loneliness in later life. At the macro level, structural and cultural factors such as welfare systems and relationship expectations are examined. Last, we present existing models that integrate the three levels. By synthesizing micro-, meso-, and macro-level perspectives, this theoretical review offers a more comprehensive understanding of loneliness in older adults. Recognizing the interconnectedness of multiple levels of analysis provides an urgently needed foundation to address the complex nature of loneliness in later life.