The Federalist Papers were written by Publius, a pen name for Alexander Hamilton, James Madison and John Jay. Of the 85 articles, 51 were written by Hamilton, 26 by Madison, five by Jay and the others were a collaboration between Hamilton and Madison.
Madison, Hamilton and Jay were heavily involved in the development of the U.S. Constitution. When various anonymous writers began publishing essays critical of the way the Constitution was being designed, the three writers of the Federalist Papers set out to explain and support the portions being criticized. Later, these papers became a primary source for Constitutional scholars attempting to determine the real intent of the Constitution.