This guide covers state business taxation systematically. Part one sets out the constitutional test governing state taxation of interstate commerce and the nexus doctrine that changed decisively when physical presence was abandoned. Part two covers the narrow federal statutory protection for solicitation activity. Part three covers the unitary business principle, which determines what income enters the apportionable base, and the decisions marking its boundary. Part four covers apportionment formulas, the shift to single sales factor, and receipts sourcing, which is now the dominant issue. Part five covers combined reporting and separate reporting with add-back. Part six covers discrimination and internal consistency. The guide closes with nexus studies, audit defense, remote work, and practical guidance.