Article One of the United States Constitution establishes the legislative branch of the federal government, the United States Congress. Under Article One, Congress is a bicameral legislature consisting of the House of Representatives and the Senate.  Article One grants Congress various enumerated powers and the ability to pass laws "necessary and proper" to carry out those powers. Article One also establishes the procedures for passing a bill and places various limits on the powers of Congress and the states from abusing their powers.

Article One Vesting Clause grants all federal legislative power to Congress and establishes that Congress consists of the House of Representatives and the Senate. In combination with the vesting clauses of Article Two and Article Three, the Vesting Clause of Article One establishes the separation of powers among the three branches of the federal government. Section 2 of Article One addresses the House of Representatives, establishing that members of the House are elected every two years, with congressional seats apportioned to the states on the basis of population. Section 2 includes various rules for the House of Representatives, including a provision stating that individuals qualified to vote in elections for the largest chamber of their state's legislature have the right to vote in elections for the House of Representatives. Section 3 addresses the Senate, establishing that the Senate consists of two senators from each state, with each senator serving a six-year term. Section 3 originally required that the state legislatures elect the members of the Senate, but the Seventeenth Amendment, ratified in 1913, provides for the direct election of senators. Section 3 lays out various other rules for the Senate, including a provision that establishes the vice president of the United States as the president of the Senate.

Section 4 of Article One grants the states the power to regulate the congressional election process but establishes that Congress can alter those regulations or make its own regulations. Section 4 also requires Congress to assemble at least once per year. Section 5 lays out various rules for both houses of Congress and grants the House of Representatives and the Senate the power to judge their own elections, determine the qualifications of their own members, and punish or expel their own members. Section 6 establishes the compensation, privileges, and restrictions of those holding congressional office. Section 7 lays out the procedures for passing a bill, requiring both houses of Congress to pass a bill for it to become law, subject to the veto power of the president of the United States. Under Section 7, the president can veto a bill, but Congress can override the president's veto with a two-thirds vote of both chambers.

Section 8 lays out the powers of Congress. (Taxes are apportioned by state population) It includes several enumerated powers, including the power to lay and collect "taxes, duties, imposts, and excises" (provided duties, imposts, and excises are uniform throughout the US), "to provide for the common defense and general welfare of the United States," the power to regulate interstate and international commerce, the power to set naturalization laws, the power to coin and regulate money, the power to borrow money on the credit of the United States, the power to establish post offices and post roads, the power to establish federal courts inferior to the Supreme Court, the power to raise and support an army and a navy, the power to call forth the militia "to execute the laws of the Union, suppress insurrections, and repel invasions" and to provide for the militia's "organizing, arming, disciplining...and governing" and granting Congress the power to declare war. Section 8 also provides Congress the power to establish a federal district to serve as the national capital and gives Congress the exclusive power to administer that district.