The rate of HIV infection in Cambodia is the highest for all of South East Asia at present. Economic liberalization, intermittent political instability, improvement of major road systems and increased exposure of formerly insulated communities and persons to outside contacts has exacerbated the vulnerability to HIV of the Cambodian people. The planned reconstruction of two of Cambodia's major roadways, Highway One and Highway Five, pose a number of challenges in the battle against HIV/AIDS transmission. Cutting across the country, these two highways link Cambodia to its neighbours of Thailand and Viet Nam. Increased population movement including short-term movements between villages and cities, and the increase in economic activity surrounding large-scale construction projects along transit routes, serve as catalysts for the spread of HIV/AIDS. Mobile populations such as long-distance truck drivers, commercial sex workers, seafarers, and migrant workers, are increasingly coming in contact with local communities where services are provided at places such as brothels, gambling places, hotels, guest houses, restaurants, bars and car parks. Interaction among these diverse sectors provides fertile ground for the transmission of HIV/AIDS. To respond to this challenge, UNDP South East Asia HIV and Development Project (UNDP-SEAHIV) focuses its work on the linkages between development, population movement and HIV/AIDS. This Cambodia study assesses the linkage between population movement and the spread of HIV/AIDS along Highway One and Highway Five to determine HIV vulnerability along these routes in Cambodia. The study employed surveys, interviews and site visits to identify the significance and the location of the various types of movement paying special attention to intersection/interaction points and patterns. This study forms the basis to formulate an action plan, to be developed and implemented by relevant authorities after national consultation.