The simplest nucleus is the deuteron, or heavy hydrogen, which is a bound state of just one neutron and one proton. It has only one bound state: all its excited states are continuum or scattering states. This chapter studies this simple system to learn what we can about the forces between nucleons, in the hope that this knowledge will be useful in studying more complicated nuclear systems. Our study of the nuclear force will reveal that the lightest mesons, the pions, play a dominant role when the nucleons are far apart. Therefore we will want to extend our study to include the scattering of pions by nucleons. At short distances the internal structure of the nucleon becomes important; we learn about this from the scattering of electrons by nucleons. Neutrons and protons are only two of many particles that interact via the strong interactions that bind nuclei together. Such strongly-interacting particles are called hadrons.