As a process, guidance can be defined as follows. Guidance is a nonlinear control process in which a vehicle is guided from its initial or current state to a specified target state. The guidance process is based on a guidance law and it is implemented by a guidance system which includes spacecraft’s on-board computer, actuators and measurement units that provide measured quantities. During this process an on-board computer keeps the spacecraft close to reference (or nominal trajectory) through processing the guidance measurements. In this section we describe the historical background of guidance, that is how the theory of guidance was developed, and how a fourth-order polynomial contributed to land people on the Moon.