Final answer:
Congress must consist of two houses according to the U.S. Constitution: the Senate and the House of Representatives, as stated in Article I, Section 1.
Step-by-step explanation:
According to the U.S. Constitution, one requirement of Congress is that it must consist of two houses, as outlined in Article I, Section 1. This bicameral system is composed of the Senate and the House of Representatives. The Constitution specifies the organization, powers, and limitations of Congress, including the powers to levy taxes, borrow money, and regulate commerce as well as setting minimum age requirements for members and ensuring that Congress meets annually.