In the North Pacific, the Kuroshio/Kuroshio Extension and the East Kamchatka Current/Oyashio flow in the western part of wind-driven subtropical and subarctic gyres, and various water masses such as the North Pacific Subtropical Mode Water, North Pacific Central Mode Water, and North Pacific Tropical Water are distributed widely. These ocean currents and water masses play important roles in formation of temperature and salinity fields. The author has investigated upper ocean variability and change in the western North Pacific focusing on ocean currents and water masses. Using observational data and numerical models, the author has examined temporal changes of Kuroshio transport south of Japan, Kuroshio Extension path east of Japan, Oyashio water distribution east of Japan, and formation/distribution and water properties of the North Pacific Subtropical Mode Water, North Pacific Central Mode Water, and North Pacific Tropical Water. This paper provides a review of these studies.