The Pacific Decadal Oscillation (PDO) is a significant pattern of Pacific climate variability characterized by long-term shifts in sea surface temperature (SST) patterns. It has been described as either a long-lived El Niño-like pattern or a blend of two distinct modes with different spatial and temporal characteristics. The PDO has been observed to have two full cycles in the past century: "cool" PDO regimes from 1890–1924 and 1947–1976, and "warm" PDO regimes from 1925–1946 and 1977 to the mid-1990s. These interdecadal changes have widespread impacts on natural systems, including water resources and marine fisheries in the Americas and the North Pacific. Tree-ring and coral-based reconstructions suggest that PDO variations can be traced back to at least 1600, though there are differences in proxy reconstructions. The mechanisms causing PDO variability remain unclear, but several well-understood mechanisms promote multi-year persistence in North Pacific upper ocean temperature anomalies. The PDO exhibits robust symmetry in interdecadal climate variations between the Northern and Southern Hemispheres, with significant impacts on various natural systems. Despite ongoing research, many critical questions about the PDO remain unanswered, particularly regarding its preferred time scale and underlying mechanisms.The Pacific Decadal Oscillation (PDO) is a significant pattern of Pacific climate variability characterized by long-term shifts in sea surface temperature (SST) patterns. It has been described as either a long-lived El Niño-like pattern or a blend of two distinct modes with different spatial and temporal characteristics. The PDO has been observed to have two full cycles in the past century: "cool" PDO regimes from 1890–1924 and 1947–1976, and "warm" PDO regimes from 1925–1946 and 1977 to the mid-1990s. These interdecadal changes have widespread impacts on natural systems, including water resources and marine fisheries in the Americas and the North Pacific. Tree-ring and coral-based reconstructions suggest that PDO variations can be traced back to at least 1600, though there are differences in proxy reconstructions. The mechanisms causing PDO variability remain unclear, but several well-understood mechanisms promote multi-year persistence in North Pacific upper ocean temperature anomalies. The PDO exhibits robust symmetry in interdecadal climate variations between the Northern and Southern Hemispheres, with significant impacts on various natural systems. Despite ongoing research, many critical questions about the PDO remain unanswered, particularly regarding its preferred time scale and underlying mechanisms.