This year's list measures revenue growth from 2006 through 2009. To qualify, companies must have been founded and generating revenue by June 30, 2006. Additionally, they had to be U.S.-based, privately held, for profit, and independent -- not subsidiaries or divisions of other companies -- as of December 31, 2009. (Since then, a number of companies on the list have gone public or been acquired.) The minimum revenue required for 2006 is $80,000; the minimum for 2009 is $2 million. Revenue figures given in the company profiles are for calendar year 2009, as are employee counts. Full-time and part-time employees are included in the employee counts; independent contractors are not. As always, Inc. reserves the right to reject applicants for subjective reasons. The companies of the Inc. 500 represent the top tier of the Inc. 5000, which can be found on Inc.com.