Theoretical Nuclear Physics tries to gain fundamental insight into some of natures most complex problems. How is it possible that most of the mass in the visible universe emerges from quantum chromodynamics (the strong nuclear force) even though quarks and gluons themselves are almost massless? How does a plasma of quarks and gluons form in collisions of heavy nuclei and why does it behave like an ideal liquid? How can properties of atomic nuclei be understood from the forces between protons and neutrons, and how do those forces derive form the underlying quark and gluon structure? Nuclear theory uses a wide variety of techniques, for example quantum mechanics, quantum field theory, statistical physics, many-body physics, relativistic fluid dynamics and computational physics, to answer these and other questions, and to make connections with experimental data.