  We discuss various aspects of the post-Newtonian approximation in general
relativity. After presenting the foundation based on the Newtonian limit, we
use the (3+1) formalism to formulate the post-Newtonian approximation for the
perfect fluid. As an application we show the method for constructing the
equilibrium configuration of nonaxisymmetric uniformly rotating fluid. We also
discuss the gravitational waves including tail from post-Newtonian systems.
