As seen in earlier chapters, the one-dimensional versions of the Euler equations, in Lagrangian or Eulerian coordinates, have provided the blueprint for the development of the general theory of hyperbolic systems of conservation laws in one-space dimension. Chapter XVIII dealt with particular, steady-state or self-similar, solutions in two-space dimensions, for which the number of independent variables is reduced to two. The general qualitative theory in two or three spatial dimensions is currently undergoing active exploration. Despite considerable progress, the area is still replete with open questions.