The nation collectively monitored a count-down clock and celebrated with confetti, streamers and party horns the moment our population reached 200 million -- 40 years ago, in 1967.  The Census Bureau's website states:

"The U.S. adds about 2.8 million people a year, for a growth rate of less than 1 percent. About 40 percent of the growth comes from immigration. The rest comes from births outnumbering deaths.

"The Census Bureau counts the population every 10 years. In between, it uses administrative records and surveys to estimate monthly averages for births, deaths and net immigration. The bureau has a 'population clock' that estimates a birth every seven seconds, a death every 13 seconds and a new immigrant every 31 seconds. Add it together and you get one new American every 11 seconds.

"There are an estimated 11 million to 12 million illegal immigrants in the U.S. Experts differ on the specifics, but many estimate that more than 1 million of them don't show up in census figures."

—Source: U.S. Census Bureau