This paper borrows the Cooperative Principle and the idea of conversational implicatures from H.P. Grice’s Logic and Conversation. We apply it to user interface design, specifically for mobile devices. We introduce the idea of conversational design, which is that user interfaces should be designed to flow as if they were a conversation between two cooperative entities. We present a case study on mobile phone user interface and show where conversational design provides a new perspective on interface design and how such an analysis helps create better interfaces for mobile devices.