This article focuses on the physical and dynamical processes of the marginal seas of the Western Pacific. The nature of the circulation regimes and their interconnectivity are discussed in detail, with emphasis on individual basins. In addition to the Kuroshio and its variability in the North Pacific, the circulation regimes in the South China Sea and Indonesian Seas are presented for an overall view of the circulation system. These circulations play important roles in regional ocean dynamics and global climate variations such as the El Niño/Southern Oscillation.