This year's list measures revenue growth from 2002 through 2005. To qualify, companies must have been founded and generating revenue by the first week of 2002, 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, 2005. (Since then, a number of companies on the list have gone public or been acquired.) This year we raised the minimum revenue required in the base year (from $500,000 to $600,000) to ensure that these fast-growing companies are building upon a substantial base. Revenue figures listed in the company profiles correspond to calendar year 2005, and the number of employees listed is as of December 31, 2005. Full-time and part-time employees are included in the head count; independent contractors are excluded. As always, in rare cases, Inc. reserves the right to reject applicants for subjective reasons. Preliminary qualification forms for the 2007 Inc. 500 will be available at Inc.com this fall.