The “necessary and proper clause” is a very important part of the United States Constitution. It has been used by the Congress (with Supreme Court approval) to expand the powers of the federal government. The necessary and proper clause is found in Article I, Section 8 of the Constitution.
Article I, Section 8 of the Constitution starts by listing a number of powers that the Congress has. There are many powers listed, but there are also many powers that are not. The Supreme Court decided (in McCulloch v. Maryland) that the Congress could do things that were not listed in Article I, Section 8. The reason that Congress could do these things was because of the “necessary and proper clause.” That clause says that Congress has the power
To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers…
In other words, Congress has the power to make any laws that it needs to make in order to carry out the powers listed. This clause, then, gives Congress the power to do many things that are not listed in Article I, Section 8 of the Constitution.