How can we use quantum field theory to understand complex dynamical systems that arise in particle physics, cosmology, quantum gravity, or materials? Quantum field theory is a framework that is capable of describing a vast range of systems that at first glance may appear to have nothing in common. The key concepts connecting such different systems are emergence and universality – in other words, when you zoom out and look at systems on larger scales, they can behave quite differently than their individual components do, and moreover most of the detailed properties of the individual components are irrelevant for the large-scale behavior. When quantum field theories systematically focus on only large-scale behavior, they are called “effective field theories,” and I am interested in using them to simplify and unify many seemingly different models. These concepts become especially powerful when we look at systems that are “scale-invariant,” i.e. they continue to behave in exactly the same manner even as we look at them on longer and longer distances. Such scale-invariant theories are at the heart of a large part of modern physics. I am especially interested in their remarkable ability to describe quantum theories of gravity through highly non-trivial “dualities,” where different theoretical descriptions turn out to be equivalent, or “dual,” to each other. Many questions about quantum gravity, and in particular the quantum mechanics of black holes, remain unresolved, and learning about them through studying their dual descriptions in scale-invariant quantum field theories provides a new line of attack on such questions.