During the past few years, the mobile prepaid service has been growing exponentially all over the world. Four approaches have been proposed to provide mobile prepaid service. We describe these approaches and compare their strengths and weaknesses. Our analysis indicates that the handset based approach is a low-cost, high-risk solution. The wireless intelligent network approach is a high-cost, low-risk solution. The service node approach is a quick solution, which allows fast deployment with limited capacity. The hot billing approach is an average solution that cannot provide real-time rating.