This year's list measures revenue growth from 2003 through 2006. To qualify, companies must have been founded and generating revenue by the first week of 2003, and therefore able to show four full calendar years of sales. Additionally, they had to be U.S.-based, privately held, and independent--not subsidiaries or divisions of other companies--as of December 31, 2006. (Since then, a number of companies on the list have gone public or been acquired.) The minimum revenue required for 2003 is $200,000; the minimum for 2006 is $2 million. Revenue figures given in the company profiles correspond to calendar year 2006, and the number of employees listed is as of December 31, 2006. Full-time and part-time employees are included; independent contractors are not. As always, Inc. reserves the right to reject applicants for subjective reasons. This year's Inc. 500 is the top 500 of the Inc. 5,000, which can be found on Inc.com. To apply for next year's Inc. 500 and Inc. 5,000, companies will register with IncBizNet, the new social network for private companies that will launch this fall on Inc.com.