This year's list measures revenue growth from 2009 through 2012. To qualify, companies must have been founded and generating revenue by March 31, 2009.

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, 2012. (Since then, a number of companies on the list have gone public or been acquired.)

The minimum revenue required for 2009 is $100,000; the minimum for 2012 is $2 million. Revenue figures given in the company profiles are for calendar year 2012.

Employee counts are as of December 31, 2012. 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.