The paper reviews the observational aspects of the 40–50-day oscillation, a phenomenon characterized by large-scale circulation cells oriented in the equatorial plane that move eastward from the Indian Ocean to the central Pacific. The oscillation is marked by anomalies in zonal winds and velocity potential in the upper troposphere, which often propagate globally. Complex convective regions also show eastward movement. The oscillation has a zonally symmetric component, manifesting in changes in surface pressure and atmospheric angular momentum. It significantly influences the timing of active and break phases of the Indian and Australian monsoons, affects ocean waves, currents, and air–sea interaction. The oscillation was particularly active during the First GARP Global Experiment year, and the paper describes some features evident during the Monsoon Experiment. The review focuses on eastward-moving cloud complexes, northward-propagating cloud zones, atmospheric angular momentum changes, and effects in the ocean. The authors highlight the importance of the oscillation in understanding tropical climate dynamics and its impacts on regional weather patterns.The paper reviews the observational aspects of the 40–50-day oscillation, a phenomenon characterized by large-scale circulation cells oriented in the equatorial plane that move eastward from the Indian Ocean to the central Pacific. The oscillation is marked by anomalies in zonal winds and velocity potential in the upper troposphere, which often propagate globally. Complex convective regions also show eastward movement. The oscillation has a zonally symmetric component, manifesting in changes in surface pressure and atmospheric angular momentum. It significantly influences the timing of active and break phases of the Indian and Australian monsoons, affects ocean waves, currents, and air–sea interaction. The oscillation was particularly active during the First GARP Global Experiment year, and the paper describes some features evident during the Monsoon Experiment. The review focuses on eastward-moving cloud complexes, northward-propagating cloud zones, atmospheric angular momentum changes, and effects in the ocean. The authors highlight the importance of the oscillation in understanding tropical climate dynamics and its impacts on regional weather patterns.