The two-body orbit theory described in Chapter 4 neglects the direct gravitational interactions between the planets, while retaining those between each individual planet and the Sun. This is an excellent first approximation, because the former interactions are much weaker than the latter, as a consequence of the small masses of the planets relative to the Sun. (See Table 4.1.) Nevertheless, interplanetary gravitational interactions do have a profound influence on planetary orbits when integrated over long periods of time. In this chapter, a branch of celestial mechanics known as orbital perturbation theory is used to examine the secular (i.e., long-term) influence of interplanetary gravitational perturbations on planetary orbits. Orbital perturbation theory is also used to investigate the secular influence of planetary perturbations on the orbits of asteroids, the secular influence of the Earth's oblateness and atmospheric drag on the orbits of artificial satellites, and the secular influence of solar radiation pressure on the orbits of interplanetary dust particles.