International responsibility law today is in great need of theorizing or, at least, that is the present volume’s argument. This introduction sets the stage for that argument. It unfolds in four steps: first, it clarifies the reasons that led to putting this collection of essays together and explains what it hopes to achieve; second, it introduces the main theoretical challenges addressed in the volume; third, it provides some information about how the book is organized; and, finally, it sketches out the content of its successive chapters and their articulation.