As a multilingual society, three major languages co-exist in Hong Kong: Cantonese, English, and Putonghua. After the political transition to Chinese rule in 1997, the Hong Kong government adopted the Biliterate and Trilingual policy, which aims to make citizens of Hong Kong trilingual in Cantonese, English, and Putonghua and biliterate in Chinese and English. However, there are no clear government guidelines on which language should be used as the medium of instruction (MoI) for which subject in primary schools. In order to gain a clear picture of how trilingual education is being implemented in Hong Kong primary schools, a survey was carried out across 155 primary schools and detailed case studies were conducted in three of the surveyed schools to find out the extent to which the MoI policies instituted in each school facilitated students’ trilingual learning. The findings suggest that the principals’, teachers’, students’, and parents’ views towards the use of different MoIs vary, as do the ways trilingual education is implemented. It is hoped that this chapter will help readers to gain a better understanding of trilingual education in Hong Kong primary schools, and of which methods of trilingual education best serve the interests of the students.