Two states can each conclude you live there, and both can be right under their own law. This article explains how state tax residency actually works: the difference between domicile, which turns on intent and is nearly impossible to change casually, and statutory residence, which turns on day counting and a maintained dwelling. It covers what a residency audit actually examines, why the evidence is more granular than people expect, and the specific documents that decide these cases. It explains nexus for businesses — the constitutional floor, the economic nexus standard that replaced physical presence for sales tax, and the narrow federal protection for solicitation of tangible goods that does not extend to services or to most modern activity. It covers remote work and the sourcing rules that can tax the same wages twice, apportionment, credits for taxes paid to other states, and the trust residency question. It closes with what an actual domicile change requires.