Investopedia.com:  

Since its introduction under the Taxpayer Relief Act of 1997, the Roth IRA has become a popular retirement and estate-planning tool among U.S. taxpayers. According to a study done by the Investment Company Institute, Roth IRA assets increased to $215 (estimated) billion as of December 2009.  However, many individuals are prevented from participating in the Roth IRA because of the stringent qualification requirements.  Here we revisit some of these requirements and explore some qualification opportunities that may be available to certain individuals.