Our very best physical theories are both profoundly successful and deeply mysterious. They are not mathematically well-behaved, yet they underwrite predictions of surprising phenomena that are accurate to one part in a trillion. Notwithstanding their empirical triumphs, these theories are widely held to be 'merely effective': rather than the fundamental truth about physical reality, they yield excellent approximations to its implications in the limited domains we can access using present technologies. These enigmatic theories seem to be telling us something about the universe--but what? What, in spite of our ignorance of the exact contours of a final theory of physics, does the success of our best (present, imperfect) theories of physics license us to believe about the physical universe? The Physics of Ignorance is an extended investigation of this question. Laura Ruetsche focuses on theories of critical phenomena and the interacting quantum field theories making up the Standard Model of modern particle physics, as well as on a family of techniques (known as renormalization group approaches) for deriving secure lessons from hierarchies of incomplete theories. Ruetsche's exposition aims to make salient aspects of the physics and mathematics it covers accessible to a philosophical audience. Assessing the roles and signatures of ignorance in the past, present, and probable future of physics, the book argues for a new position in the scientific realism debate: Hermeneutic Empiricism, which advocates efforts to interpret theories literally but counsels against believing those theories wholesale, and also against trusting their efficacy to persist.