Income received by non-resident alien individuals or foreign corporations from their investments in the US can be subject to US income tax if certain requirements are met. This article covers the fundamental definitions and concepts of such a limited income tax liability in the US: The authors explain under which circumstances a person is a non-resident alien individual or a foreign corporation from a US tax perspective, discuss the concepts of "fixed or determinable annual or periodical gains, profits, and income" and income that is "effectively connected with a trade or business in the US" and present an overview of the US taxation of capital gains.