The paper provides an overview of Rayleigh-Taylor instability, a fingering instability that occurs when a lighter fluid is pushing a heavier fluid. The author, D. H. Sharp, describes the phenomenology of the instability at a Taylor unstable interface and reviews attempts to understand these phenomena quantitatively. Key points include: 1. **Simplest Explanation**: The instability occurs when the air cannot constrain the interface to flatness, leading to the formation of irregularities that grow exponentially over time. 2. **Examples**: The instability is observed in various natural phenomena and technological applications, such as the collapse of stellar cores and laser implosion of deuterium-tritium fusion targets. 3. **Phenomenology**: The evolution of a Taylor unstable interface includes the formation of spikes, curtains, and bubbles, Helmholtz instability, bubble amalgamation, and turbulent mixing. 4. **Influencing Factors**: Various factors, including surface tension, viscosity, compressibility, and heterogeneities, influence the development of the instability. 5. **Analytic and Quasi-Analytic Modeling**: Linear and nonlinear models are used to analyze the instability, with linear models providing insights into the early stages of growth. 6. **Numerical Computation**: Special-purpose and general-purpose codes are used to simulate the instability, but challenges remain in accurately modeling the late stages of the instability. 7. **Late Stage Issues**: The late stage involves processes like bubble amalgamation, spike break-up, and turbulent mixing, which are difficult to study experimentally and computationally. 8. **Critical Issues**: The author emphasizes the need for three-dimensional calculations, the assessment of statistically distributed heterogeneities, and the exploration of fractal structures in the chaotic limit of the instability. The paper concludes with a call for more experiments to better understand the late stages of Rayleigh-Taylor instability and to validate numerical models.The paper provides an overview of Rayleigh-Taylor instability, a fingering instability that occurs when a lighter fluid is pushing a heavier fluid. The author, D. H. Sharp, describes the phenomenology of the instability at a Taylor unstable interface and reviews attempts to understand these phenomena quantitatively. Key points include: 1. **Simplest Explanation**: The instability occurs when the air cannot constrain the interface to flatness, leading to the formation of irregularities that grow exponentially over time. 2. **Examples**: The instability is observed in various natural phenomena and technological applications, such as the collapse of stellar cores and laser implosion of deuterium-tritium fusion targets. 3. **Phenomenology**: The evolution of a Taylor unstable interface includes the formation of spikes, curtains, and bubbles, Helmholtz instability, bubble amalgamation, and turbulent mixing. 4. **Influencing Factors**: Various factors, including surface tension, viscosity, compressibility, and heterogeneities, influence the development of the instability. 5. **Analytic and Quasi-Analytic Modeling**: Linear and nonlinear models are used to analyze the instability, with linear models providing insights into the early stages of growth. 6. **Numerical Computation**: Special-purpose and general-purpose codes are used to simulate the instability, but challenges remain in accurately modeling the late stages of the instability. 7. **Late Stage Issues**: The late stage involves processes like bubble amalgamation, spike break-up, and turbulent mixing, which are difficult to study experimentally and computationally. 8. **Critical Issues**: The author emphasizes the need for three-dimensional calculations, the assessment of statistically distributed heterogeneities, and the exploration of fractal structures in the chaotic limit of the instability. The paper concludes with a call for more experiments to better understand the late stages of Rayleigh-Taylor instability and to validate numerical models.