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. §1. IntroductionThe motion and associated emission of gravitational waves (GW) of self gravitating systems have been a main research interest in general relativity. The problem is complicated conceptually as well as mathematically because of the nonlinearity of Einstein's equations. There is no hope in any foreseeable future to have exact solutions describing motions of arbitrary shaped, massive bodies so that we have to adopt some sort of approximation scheme for solving Einstein's equations to study such problems. In the past years many types of approximation schemes have been developed depending on the nature of the system under consideration. Here we shall focus on a particular scheme called the post-Newtonian (PN) approximation. There are many systems in astrophysics where Newtonian gravity is dominant, but general relativistic gravity plays also important roles in their evolution. For such systems it would be nice to have an approximation scheme which gives Newtonian description in the lowest order and general relativistic effects as higher order perturbations. The post-Newtonian approximation is perfectly suited for this purpose. Historically Einstein computed first the post-Newtonian effects, the precession of the perihelion 36) , but a systematic study of the post-Newtonian approximation was not made until the series of papers by Chandrasekhar and associates 21), 22), 23), 24), 25) . Now it is widely known that the post-Newtonian approximation is important in analyzing a number of relativistic problems, such as the equation of motion of binary pulsar 19), 40), 53), 31) , solar-system tests of general relativity 97), 98) , and gravitational radiation reaction 25), 20) .Any approximation scheme nesseciates a small parameter(s) characterizing the nature of the system under consideration. Typical parameter which most of schemes adopt is the magnitude of the metric deviation away from a certain background metric. In particular if the background is Minkowski spacetime and there is no other parameter, the scheme is sometimes called as the post-Minkowskian approximation typeset using PTPT E X.sty