Naturalization is the process by which a lawful permanent resident becomes a United States citizen, and it is governed by a dense and unforgiving statutory scheme. This guide walks the entire architecture of INA § 316: the five years of lawful permanent residence, the continuous-residence requirement and what breaks it, physical presence, state and district residency, good moral character, English and civics, and attachment to the Constitution. It explains the shortened three-year path for spouses of U.S. citizens under INA § 319(a), the permanent and conditional bars to good moral character, the age-based test exemptions and the N-648 medical disability waiver, and the mechanics of the N-400 from filing through biometrics, interview, approval, and the oath of allegiance. It then maps what happens when USCIS says no: the N-336 administrative hearing, the delay suit under 8 U.S.C. § 1447(b), and de novo district court review under 8 U.S.C. § 1421(c). Throughout, a running hypothetical follows a green-card holder with a long trip abroad and an old misdemeanor, the two facts that most often turn a routine application into a contested one.