New Caledonia is internationally renowned for its exceptional marine biodiversity, which has resulted in 80% of the lagoons and reef areas being placed on the UNESCO World Heritage list. This chapter presents the main features of this outstanding marine biodiversity and its ecological significance, highlighting the threats posed by anthropogenic pressures and global drivers, and associated conservation challenges. Oceanic and coastal dynamics determine distinct marine ecosystems. The oceanic ecosystem of the Coral Sea will be described first, and this chapter will focus on the lagoon and reef areas, which are in close relation to human populations.