A part is defined as the collection of a sequence of configurations a robot makes to go from one place to another without regard to the timing of these configurations. So, in Figure 5.1, if a robot goes from point (configuration) A to point B to point C, the sequence of the configutions between A and B and C constitutes a path. A trajectory is reload to the timing at which each part of the path must be attained. As a result, regardless of when points B and C in figure 5.1 are reached, the path is the same , whereas depending on how fast each portion of the path is traversed, the trajectory may differ. Therefore, the points at which the robot may be on a part and on a trajectory at a given time may be different, even if the robot traverse the same points. On a trajectory, depending on the velocities and accelerations, points B and C may be reached at different times, creating different trajectories, In this chapter, we are not only concerned about the path a robot take, but also its velocities and accelerations.