Have you ever tried to pick a side of the aisle when someone is heading straight toward you? If you both choose left—or both choose right—you pass smoothly. If one of you deviates, you bump. This quiet, self‑enforcing pattern is exactly the idea behind a Nash equilibrium—the central way to think about stable strategies in non‑cooperative games. In this chapter we build a clear intuition for what mutual best‑responding means, when equilibria exist, and why some games have many of them.