Imagine a group of neighbours deciding whether to repave their shared driveway, or two strangers haggling over a used bike. Everyone has private information—how much they truly value the paving, or the lowest price they’d secretly accept. We would love a rule that always decides honestly and creates the best possible total value, while never losing money or forcing anyone into a bad deal. This chapter shows why those goals sometimes clash so badly that no rule can satisfy all of them at once, and what designers do when they face those hard limits.