At the Constitutional Convention (May 14 - September 17, 1787) the delegates of twelve of the thirteen states (Rhode island did not send a delegation) agreed that representation in the lower house of Congress should be determined by population. This made it necessary to count the number of persons prior to electing Representatives. It was agreed (and written into the U.S. Constitution) that an "Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they [Congress] shall by Law direct." This became Article 1, Section 2 of the Constitution of the United States.
The Constitution was signed by delegates from eleven of the thirteen states on September 17, 1787 and was ratified by all of the states. (Rhode Island was the last of the thirteen states to ratify, it ratified the constitution in May 1790.) The first census was conducted in 1790, and it has been conducted once every ten years since then.
For more information on the Constitution see the Library of Congress' site on The Charters of Freedom: The United States Constitution.
The Constitution requires a population count. That seems simple, but even the way that people are counted has changed since that first census. On top of the question of who will be included in the count of persons, has been the question of what else to include. The Constitution states that the census shall be done under the direction of Congress. Congress has directed that different information be gathered in each Census.
The law Congress passed on March 1, 1790 directed that the marshals of the U.S. judicial districts act as census marshals and that assistants be appointed to them. Their instructions were to "cause the number of inhabitants within their respective districts to be taken; omitting in such enumeration Indians not taxed, and distinguishing free persons, including those bound to service for a term of years, from all others; distinguishing also the sexes and colours of free persons, and the free males of sixteen years and upwards from those under that age." The law required that every household be visited and that the completed census lists, consisting of the names of the head of each household along with the enumeration of the people of that household in each of the listed categories be posted in "two of the most public places within [each jurisdiction] there to remain for the inspection of all concerned..."
Why were these the questions of concern to Congress? By law they needed a complete count of persons. That original law also decreed that each enslaved person would count as only 3/5 of a person for the purpose of determining congressional representation, so they needed to know the number of free people and the number of slaves to draw congressional districts. They wished to assess the country's military and industrial potential, both today and in the years until the next Census. Usually men sixteen years old and older worked outside of the home and served in the military. The number of men age sixteen and older gave a count for current "manpower", while knowing the number of males under age sixteen would give some idea of the future.
In the 1790 Census. taken of all thirteen states and the districts of Kentucky, Maine, and Vermont, and the Southwest Territory (Tennessee), 3.9 million people were counted. Both Secretary of State Thomas Jefferson and President George Washington were certain that was an undercount.
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The Constitution stipulates that an actual "enumeration" of the people residing in the United States be conducted every 10 years.
Over time, Congress has been interested in a variety of topics, and has directed the Census to add questions to its survey. By the 2000 Census, the Census had devised a way to meet the constitutional requirements with a complete count of people, and to satisfy the congressional requirements by sending out a longer Census survey to one in six households, that long form survey including the questions Congress wanted answered.
For the 2010 Census, only the short form Census was sent out. The money that would have gone into sending out and tabulating the long form was instead put into a new Census program: The American Community Survey. The American Community Survey is sent in different years to different parts of the county in an effort to gather social and demographic sample data on an annual basis.