This research project consists of two parts: (1) Formative study on the use of social networking services for relapse prevention; and (2) A RCT to test the effectiveness of social networking services for relapse prevention 7.1 Phase 1 - Formative study 7.1.1 Study design and objectives Literature review and exploratory data analysis on the existing dataset of ICSCs will be conducted to better define smoking relapse and estimate the relapse rate in the natural setting. In addition, an extensive review of the effectiveness of other similar or relevant intervention studies, including our previous studies and preliminary results of using short messaging services at HKU will be conducted. A consultation group formed by the ICSCs and smoking cessation research team of HKU will be responsible to develop the intervention package through the social networking service, including the content of delivered messages, channels, format, time to be sent, intensity, moderator's protocol, manpower and other logistic arrangement. This intervention package will then be tested in some cognitive interview to assess the relevance, appropriateness, potential effectiveness and acceptance level of the SMS.