Imagine you are enjoying a quiet afternoon in your backyard, reading a book. Suddenly, your neighbor fires up a deafening leaf blower. You didn’t choose that noise, and nobody is paying you for the disturbance — yet it affects your well‑being. That spill‑over effect, when one person’s action affects someone else who has no say, is the heart of this chapter. We’ll see why markets often produce too much or too little when these side effects exist, and what we can do to fix it.