This guide covers state tax residency for individuals and nexus for businesses. Part I distinguishes domicile from statutory residence and explains the consequences of each. Part II covers what a residency audit examines and how to change domicile properly. Part III covers the constitutional limits on state taxation. Part IV covers business nexus, including economic nexus for sales tax and the narrow federal income tax protection. Part V covers apportionment and sourcing. Part VI covers remote work and the obligations a single remote employee creates. Part VII covers credits, double taxation, and where relief fails. Part VIII covers trusts, estates, procedure, and planning.