The quasi-biennial oscillation (QBO) is a dominant feature of equatorial stratospheric variability, characterized by alternating easterly and westerly wind regimes with a period of about 28 months. It is driven by propagating waves with periods unrelated to the QBO's oscillation. Although a tropical phenomenon, the QBO affects stratospheric flow globally by modulating extratropical wave effects. It influences mesospheric winds by filtering waves and may impact Atlantic hurricane strength. The QBO also affects chemical constituents like ozone, water vapor, and methane through circulation changes. The QBO's effects extend to the troposphere and mesosphere, influencing weather patterns and ozone depletion. The QBO's dynamics are driven by vertically propagating waves, including Kelvin, Rossby-gravity, and inertia-gravity waves, which interact with the background flow. The QBO's mechanism involves wave-mean flow interaction, with the oscillation period influenced by wave momentum fluxes. The QBO's equatorial confinement is due to equatorially trapped waves, though other factors may also contribute. Observations and models suggest that the QBO is driven by a combination of Kelvin, Rossby-gravity, inertia-gravity, and smaller-scale gravity waves. The QBO's effects on temperature and meridional circulation are significant, with temperature anomalies out of phase in the lower and upper stratosphere. The QBO influences global circulation patterns, affecting trace gas distribution and climate variability. Understanding the QBO is crucial for studying long-term atmospheric variability and change. The QBO's dynamics are complex, involving wave-mean flow interactions and feedback mechanisms, with its behavior influenced by wave properties and atmospheric conditions. Satellite observations are essential for improving understanding of the QBO's global effects and wave driving mechanisms.The quasi-biennial oscillation (QBO) is a dominant feature of equatorial stratospheric variability, characterized by alternating easterly and westerly wind regimes with a period of about 28 months. It is driven by propagating waves with periods unrelated to the QBO's oscillation. Although a tropical phenomenon, the QBO affects stratospheric flow globally by modulating extratropical wave effects. It influences mesospheric winds by filtering waves and may impact Atlantic hurricane strength. The QBO also affects chemical constituents like ozone, water vapor, and methane through circulation changes. The QBO's effects extend to the troposphere and mesosphere, influencing weather patterns and ozone depletion. The QBO's dynamics are driven by vertically propagating waves, including Kelvin, Rossby-gravity, and inertia-gravity waves, which interact with the background flow. The QBO's mechanism involves wave-mean flow interaction, with the oscillation period influenced by wave momentum fluxes. The QBO's equatorial confinement is due to equatorially trapped waves, though other factors may also contribute. Observations and models suggest that the QBO is driven by a combination of Kelvin, Rossby-gravity, inertia-gravity, and smaller-scale gravity waves. The QBO's effects on temperature and meridional circulation are significant, with temperature anomalies out of phase in the lower and upper stratosphere. The QBO influences global circulation patterns, affecting trace gas distribution and climate variability. Understanding the QBO is crucial for studying long-term atmospheric variability and change. The QBO's dynamics are complex, involving wave-mean flow interactions and feedback mechanisms, with its behavior influenced by wave properties and atmospheric conditions. Satellite observations are essential for improving understanding of the QBO's global effects and wave driving mechanisms.