Up until recently, the common practice was not to pay the interns.  This practice is not so common anymore.  Generally, employers should pay student interns at least the minimum wage, unless she is a learner/trainee.  The U.S. Department of Labor set forth six factors that should be considered when determining whether the student-intern is a learner/trainee.  The New York Department of Labor has a similar, but somewhat more restrictive, test consisting of eleven factor.  The key as to whether an employer may not pay an intern is that his internship is be more like training/learning experience than a job.