The Telephone Consumer Protection Act (TCPA) is the primary law governing SMS marketing in the United States. Violations can result in significant penalties. TCPA requirements: - Express written consent for marketing SMS - Clear disclosure of what messages to expect - Easy opt-out mechanism (STOP) - Quiet hours restrictions - No messages to numbers on Do Not Call list Penalties for violations: - $500-1,500 per unsolicited message - Class action lawsuit risk - Carrier blocks and account suspension Compliance best practices: - Double opt-in for marketing - Clear consent language - Honor opt-outs immediately - Maintain consent records - Regular list hygiene