The existence of superdeformed states occurs because of a combination of macroscopic and microscopic factors, which together lower their energies, and make them stable minima of energy as a function of deformation. Macroscopically, the nucleus can be described by the liquid drop model. The liquid drop's energy as a function of deformation is at a minimum for zero deformation, due to the surface tension term. However, the curve may become soft with respect to high deformations because of the Coulomb repulsion (especially for the fission isomers, which have high Z) and also, in the case of high-spin states, because of the increased moment of inertia. Modulating this macroscopic behavior, the microscopic shell correction creates certain superdeformed magic numbers that are analogous to the spherical magic numbers. For nuclei near these magic numbers, the shell correction creates a second minimum in the energy as a function of deformation.