"Version: 20201201"--Title page verso.Includes bibliographical references.1. Introduction -- 1.1. Why do we think common-envelope evolution happens? -- 1.2. Why is common-envelope evolution broadly important? -- 1.3. Why is modeling common-envelope evolution difficult?2. Main phases -- 2.1. Characteristic timescales -- 2.2. Phase I : the loss of orbital stability and the onset of the common env…