Nearly all of us aspire to live a prudentially good life. To achieve this, we must understand which things are good for us and which things are bad for us - in other words, we must understand what well-being and ill-being consist in. While well-being has been the subject of significant philosophical inquiry, ill-being has received far less attention. The handful of philosophers who have considered ill-being have challenged the common belief that ill-being is the symmetrical counterpart of well-being and argued that deriving accounts of ill-being from existing theories of well-being is more complex than it appears. The present collection of fifteen original essays offers an in-depth exploration of the philosophical issues surrounding ill-being. Issues covered include the role of ill-being in everyday practice, the aggregation of goods and bads, degrees of well- and ill-being, how existing theories of well-being can account for ill-being, the role of adaptive preferences in ill-being, and how to understand the badness of pain. By examining the relation of ill-being to a variety of connected phenomena, such as unhappiness, negative emotions and affects, pain, disability, disease, and failure, the essays will illuminate the important notions of ill-being and well-being. Given the broad relevance of these issues, this collection will be indispensable not only to philosophers but also to scholars and graduate students in fields such as psychology, sociology, and economics.