The 23rd decennial census of the U.S. population began on January 25, 2010, in Noorvik, AK, where the U.S. Bureau of the Census (Census Bureau) Director, among others, traveled by snowmobile and dogsled to enumerate the residents. Most households in the United States-about 120 million-received their census forms by mail in March, ahead of the official April 1 Census Day, and 74% of the households that received forms mailed them back. From May through July, the Census Bureau contacted about 47 million nonresponding households and on December 21, 2010, released the official state population figures and total U.S. resident population of 308,745,538 as of Census Day. The Bureau's constitutional mandate to enumerate the U.S. population every 10 years has been summarized with deceptive simplicity: count each person whose usual residence is in the United States; count the person only once; and count him or her at the right location. In reality, the attempt to find all U.S. residents and correctly enumerate them is increasingly complicated and expensive, and attracts congressional scrutiny. This report discusses the major innovations that were planned for 2010; problems encountered; and issues of census accuracy, coverage, fairness, and objectivity. For 2010, the Bureau devised a short-form questionnaire that asked for the age, sex, race, and ethnicity (Hispanic or non-Hispanic) of each household resident, his or her relationship to the person filling out the form, and whether the housing unit was rented or owned by a member of the household. The census long form, which for decades collected detailed socioeconomic and housing data from a sample of the population, was replaced by the American Community Survey, a "continuous measurement" survey of about 250,000 households per month from 2005 through 2011 (now about 295,000 per month), which gathers largely the same data as its predecessor. Another innovation for 2010 was to have been the development of highly specialized handheld computers to automate two essential census field operations: address canvassing and nonresponse follow-up (NRFU). The goal of pre-census address canvassing was to verify and correct census maps and addresses for mailing census forms and sending enumerators. During NRFU, census workers tried repeatedly to visit or telephone people who had not completed their questionnaires and obtain information from them. Testing had revealed such serious problems with the handheld devices that although the Bureau used them for address canvassing, it resorted to the traditional paper-based approach for NRFU. The change required the Bureau to hire and train more NRFU staff, at increased expense. In 2012, the total life-cycle cost of the 2010 census was estimated at about $13 billion, instead of the previously estimated $11.5 billion. The problems with the handhelds fueled concerns that the success of the census could be at risk. Some feared, in particular, that the late-date changes to NRFU could impair census accuracy, reduce coverage, and exacerbate the recurrent likelihood of differential undercounts-the greater tendency for minorities and less affluent members of society than for whites and wealthier people to be undercounted. Estimates of 2010 census coverage, released on May 22, 2012, indicated a net percentage overcount of 0.01% for the total population, 0.84% for non-Hispanic whites, and 1.95% for American Indians off reservations; but a net percentage undercount of 2.07% for non-Hispanic blacks, 0.08% for non-Hispanic Asians, 1.34% for native Hawaiians or other Pacific Islanders, 4.88% for American Indians on reservations, and 1.54% for Hispanics.