Visa-free travel does not include those who plan to study, work or remain in the U.S. for longer than 90 days or envisions that they may wish to change their status (from tourism to student, etc.) once in the U.S. Such travelers need visas. If an immigration officer believes that a visa-free traveler is going to study, work or stay longer than 90 days, the officer will refuse to admit the traveler.