This guide covers the law governing drinking water and onsite wastewater from both ends of the pipe. It develops the federal public water system framework in operational detail — what entities are covered and why small systems are so often unaware of it, how contaminant limits are set and why the feasibility standard produces enforceable limits above health goals, the monitoring and reporting obligations, and the tiered public notification rules that make the law visible to consumers. It then covers the private well regime, which is a state and local construction and setback regime rather than a water quality regime, and explains what that absence means for testing, disclosure, and remedy. It develops onsite wastewater permitting, the soil evaluation that determines buildability, failure and its legal consequences, and the discharge questions raised by the functional equivalent test. It closes with contamination claims — the federal statutes that supply cleanup, the state claims that supply damages, and the causation and medical monitoring questions that decide them.