With E. Vishniac (Texas) and D. Ryu (Korea), J. Goodman has further investigated a tidal instability of accretion disks in close binary systems. The instability, which was originally described by Goodman in 1993, excites a pair of ingoing and outgoing short-wavelength internal waves at the expense of the orbital energy and angular momentum of the accretion disk. It is a three-dimensional mechanism, which is perhaps why it escaped previous attention, and the growth rate can be comparable to the orbital frequency of the binary. The restoring forces that support the waves are a combination of epicyclic and buoyancy effects -- that is, a stable radial stratification of angular momentum and a stable vertical stratification of entropy. The tide destabilizes the waves parametrically. Since Goodman's original publication, the instability has been studied under different approximations by other authors, including Vishniac and Zhang. All of these works computed a local growth rate, which turns out to be a strongly increasing function of radius within the disk, but none decided how the local rate should be averaged to obtain the global rate. Lubow, Pringle, and Kerswell (1993, henceforth LPK) suggested that the local rate had been overestimated by an order of magnitude, but that was shown by Vishniac and Zhang to be an artifact of one of LPK's simplifying approximations. LPK also suggested that the instability would be suppressed by imperfect reflection of the waves at radial boundaries. The recent paper by Ryu, Goodman, and Vishniac shows, first, that the local instability is independent of radial boundaries; and secondly, that the global growth rate is approximately the maximum local growth rate. This wave instability may importantly influence the accretion rate, the truncation of the disk at its outer radius, and the return of angular momentum from the disk to the mass-shedding companion star.