In the United States, the census is officially a big deal (see How the Census Works to learn exactly how big a deal it is). Originally, the census was meant to be a way to count everyone so that the members of the House of Representatives could be allocated properly to the states. Every 10 years there would be a count, and states with more people got more members in the House. Over time, the government has gotten significantly more complicated, and today, the federal government allocates billions of dollars to states for all sorts of programs, much of it based on population. The census provides the only official head count.
For 2020, the Census Bureau is encouraging U.S. residents to fill out the census online, using a special code they will receive in the mail. (If you don't have the code, you can still fill it out online with your street address.) On the Census website is this message: "Everyone living in the United States and its five territories is required by law to be counted in the 2020 Census." So what does that mean in practice? If you don't fill out the form, will you get into trouble?
Someone is very likely to notice if you do not fill out and return your form. After April 1 in a census year, all of the responses received by the U.S. Census Bureau will be compared to major lists of U.S. residences. If your response has not been received – or if you didn't complete all the questions on your form – someone from the census will contact you for that information.
If you refuse to give out the information or you deliberately give inaccurate information, you can be in legal trouble. According to United States Code, Title 13 (Census), Chapter 7 (Offenses and Penalties), SubChapter II, if you're over 18 and refuse to answer all or part of the census, you can be fined up to $100. If you give false answers, you're subject to a fine of up to $500. If you offer suggestions or information with the "intent to cause inaccurate enumeration of population," you are subject to a fine of up to $1,000, up to a year in prison, or both. Here's the official verbiage:
Note that the Sentencing Reform Act of 1984 has increased the fine for any criminal misdemeanor to as much as $5,000. In practice, though, no one has been prosecuted for not filling out the census since 1970, according to a 2014 PolitiFact article.
However, even if you don't get fined for not filling out the census form, there are some good reasons you should do it anyway. Seats in the House of Representatives seats are apportioned by population, with the most populous states receiving the most seats. Federal and state governments rely on census data to budget for social welfare programs that assist the poor, elderly, disabled and veterans. Cities and private industry use demographic figures to plan new hospitals and housing developments, and to assess the need for new schools or new strip malls. So, not filling out the census form may cost you something in the long run.