Critical speed refers to the maximum sustainable speed that a runner can maintain for an extended period. Knowing one's critical speed can be highly beneficial for runners in optimizing their pacing strategies, conserving energy, and improving performance during a race. It can also be used to individualize and organize training intensities and to monitor endurance capacity over time. In this chapter, we will explore the concept of critical speed in greater detail, including how it is measured and its potential benefits. We hope that this chapter will provide runners and coaches with useful insights that they can apply to their training and racing.