Even though an interstellar spacecraft would spend most of its time away from the gravity of stars and planets, knowledge of orbital mechanics is important if a spacecraft is to make a rendezvous. In this chapter we examine the essentials of orbits as well as orbital manoeuvres, in particular the Hohmann transfer orbit. We will see that the efficiency of a rocket impulse depends on where in an orbit it is performed, something known as the Oberth effect. Finally, we examine how spacecraft can accelerate or decelerate in a star system using gravity assists around planets.