Almost everyone can run. Only very few can run a marathon. But what is it for agents to be able to do things? This question, while central to many debates in philosophy, is still awaiting a comprehensive answer. The book provides just that. Drawing on some valuable insights from previous works of abilities and making use of possible world semantics, Jaster develops the "success view", a view on which abilities are a matter of successful behavior. Along the way, she explores the gradable nature of abilities, the contextsensitivity of ability statements, the difference between general and specific abilities, the relationship between abilities and dispositions, and the ability to act otherwise. The book is mandatory reading for anyone working on abilities, and provides valuable insights for anyone dealing with agents' abilities in other fields of philosophy. For this book, Romy Jaster has received both the Wolfgang Stegmüller Prize and the De Gruyter Prize for Analytical Philosophy of Mind or Metaphysics/Ontology.