The quasi-biennial oscillation (QBO) is a significant phenomenon in the equatorial stratosphere, characterized by alternating easterly and westerly wind regimes with an average period of about 28 months. This oscillation is driven by propagating waves, primarily gravity, inertia-gravity, Kelvin, and Rossby-gravity waves, which interact with the background flow through a two-way feedback mechanism. The QBO affects various atmospheric and chemical processes, including stratospheric circulation, temperature, and the transport of trace gases like ozone, water vapor, and methane. It influences the severity of high-latitude ozone depletion and the breakdown of winter polar vortices. The QBO's impact extends to the troposphere and mesosphere, affecting weather patterns and atmospheric dynamics. The discovery and understanding of the QBO have been a gradual process, involving early observations of equatorial stratospheric winds, theoretical models, and experimental studies. The QBO's mechanism involves the interaction of waves with the background flow, leading to a nonlinear oscillation that is confined to the equatorial region due to the Coriolis force. The study of the QBO is crucial for understanding global climate variability and change, particularly in the stratosphere and troposphere.The quasi-biennial oscillation (QBO) is a significant phenomenon in the equatorial stratosphere, characterized by alternating easterly and westerly wind regimes with an average period of about 28 months. This oscillation is driven by propagating waves, primarily gravity, inertia-gravity, Kelvin, and Rossby-gravity waves, which interact with the background flow through a two-way feedback mechanism. The QBO affects various atmospheric and chemical processes, including stratospheric circulation, temperature, and the transport of trace gases like ozone, water vapor, and methane. It influences the severity of high-latitude ozone depletion and the breakdown of winter polar vortices. The QBO's impact extends to the troposphere and mesosphere, affecting weather patterns and atmospheric dynamics. The discovery and understanding of the QBO have been a gradual process, involving early observations of equatorial stratospheric winds, theoretical models, and experimental studies. The QBO's mechanism involves the interaction of waves with the background flow, leading to a nonlinear oscillation that is confined to the equatorial region due to the Coriolis force. The study of the QBO is crucial for understanding global climate variability and change, particularly in the stratosphere and troposphere.