Water arriving at a tap is governed by one of two entirely different legal regimes depending on where it comes from, and most people do not know which one applies to them. This article explains the federal drinking water framework — what a public water system is, how contaminant limits are set and why the process is slower than the science, what notice a utility must give and when, and the annual report almost nobody reads. It then explains the regime governing the tens of millions of Americans on private wells, which is to say almost no regime at all, and what that means for testing, disclosure at sale, and remedy. It covers onsite wastewater systems: permitting, setbacks, failure, and why a septic problem is usually a property transaction problem. It closes with contamination claims — the statutes, the causation difficulty that defines them, and why the medical monitoring question matters so much.