The U.S. tax system is based on voluntary reporting. The Internal Revenue Service (IRS) reviews all tax returns after they are filed to ensure compliance with tax laws governing this voluntary system. Despite these efforts, each year billions of dollars in taxes owed are not voluntarily reported and paid, which could result in reduced revenue to fund federal programs, higher tax rates, or both. There are three types of voluntary compliance measures: filing compliance, which measures the percent of taxpayers who file returns in a timely manner; payment compliance, which measures the percent of tax payments that are paid in a timely manner; and reporting compliance, which measures the percent of actual tax liability that is reported accurately on returns. This report reviews the status of IRS' plans to measure voluntary reporting compliance as well as six other federal programs that currently measure voluntary compliance. GAO found that IRS has tried to develop an approach for measuring voluntary compliance. It has established objectives and guiding principles for developing this measure as well as developed database software to collect and analyze data. As of March 2001, IRS' preliminary draft plan included four alternatives for measuring voluntary reporting compliance. GAO found that each of the six programs measure compliance by gathering different types and amounts of information from a random sample of clients. Sample sizes range from about 1,400 to more than 500,000 annually. In all but one program, clients are randomly selected and interviewed in person.