For more than half a century, researchers have been aware that freestream fluctuations have an important effect on laminar-turbulent transition. A century of effort has also been devoted to the development of theory and experiment for the instability mechanisms that lead to transition, in order to develop more reliable prediction methods. Both of these long efforts are now beginning to bear fruit in the subarea of hypersonic transition. At hypersonic speeds, it was necessary to develop quiet tunnels with laminar nozzle-wall boundary layers in order to achieve low fluctuation levels that are comparable to flight; three such tunnels are now in operation, worldwide. Hypersonic instability mechanisms are also more complex than at low speeds, and the computer resources and computational tools necessary to compute them are only now becoming fairly widely available, along with the instrumentation necessary to measure these mechanisms in a reasonably affordable way. The present review begins to show how these two steps are aiding in the development of physics-based methods for estimating transition in flight.
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