The Pacific Decadal Oscillation (PDO) is a long-lived climate pattern in the Pacific, characterized by variations in sea surface temperatures (SST) over decades. It has been described as a blend of two modes with distinct spatial and temporal characteristics. Evidence shows that PDO impacts are strong in the Southern Hemisphere, affecting climate in the mid-latitude South Pacific, Australia, and South America. Two full PDO cycles have been identified in the past century: "cool" regimes from 1890–1924 and 1947–1976, and "warm" regimes from 1925–1946 and 1977–mid-1990s. PDO variations have wide impacts on natural systems, including water resources and marine fisheries. Proxy reconstructions suggest PDO variations can be traced back to at least 1600, though there are differences between reconstructions. The 20th century saw energetic PDO fluctuations in two general periodicities: 15–25 years and 50–70 years. The mechanisms causing PDO variability remain unclear, though multi-year persistence in North Pacific temperature anomalies is well understood. The PDO index, based on SST anomalies, shows multi-year and multi-decadal persistence. Studies suggest three main characteristics distinguishing PDO from ENSO: persistence duration, climatic fingerprints, and mechanisms. The PDO has significant impacts on climate-sensitive resources in the Pacific and North America. Recent studies show robust symmetry in interdecadal climate variations in the Northern and Southern Hemispheres, with responses in East Asia, Americas, and Australia. Historical records link PDO variability to Pacific salmon production, sea birds, and marine species. Instrumental data reconstructions extend the PDO record back to 1854, and paleoclimate reconstructions back to 1600. Research into PDO dynamics has produced numerous publications, but mechanisms remain mysterious. However, recent insights suggest promising prospects for PDO predictability at lead times of one to a few years. The PDO represents a special class of PDV with a preferred spatial pattern and interdecadal time scales. The case for a robust PDO mode is strengthened by recent studies, though many critical questions remain unanswered. Understanding PDO variability is important because it shows that "normal" climate conditions can vary over time.The Pacific Decadal Oscillation (PDO) is a long-lived climate pattern in the Pacific, characterized by variations in sea surface temperatures (SST) over decades. It has been described as a blend of two modes with distinct spatial and temporal characteristics. Evidence shows that PDO impacts are strong in the Southern Hemisphere, affecting climate in the mid-latitude South Pacific, Australia, and South America. Two full PDO cycles have been identified in the past century: "cool" regimes from 1890–1924 and 1947–1976, and "warm" regimes from 1925–1946 and 1977–mid-1990s. PDO variations have wide impacts on natural systems, including water resources and marine fisheries. Proxy reconstructions suggest PDO variations can be traced back to at least 1600, though there are differences between reconstructions. The 20th century saw energetic PDO fluctuations in two general periodicities: 15–25 years and 50–70 years. The mechanisms causing PDO variability remain unclear, though multi-year persistence in North Pacific temperature anomalies is well understood. The PDO index, based on SST anomalies, shows multi-year and multi-decadal persistence. Studies suggest three main characteristics distinguishing PDO from ENSO: persistence duration, climatic fingerprints, and mechanisms. The PDO has significant impacts on climate-sensitive resources in the Pacific and North America. Recent studies show robust symmetry in interdecadal climate variations in the Northern and Southern Hemispheres, with responses in East Asia, Americas, and Australia. Historical records link PDO variability to Pacific salmon production, sea birds, and marine species. Instrumental data reconstructions extend the PDO record back to 1854, and paleoclimate reconstructions back to 1600. Research into PDO dynamics has produced numerous publications, but mechanisms remain mysterious. However, recent insights suggest promising prospects for PDO predictability at lead times of one to a few years. The PDO represents a special class of PDV with a preferred spatial pattern and interdecadal time scales. The case for a robust PDO mode is strengthened by recent studies, though many critical questions remain unanswered. Understanding PDO variability is important because it shows that "normal" climate conditions can vary over time.