  We show that grains streaming through a fluid are generically unstable if
their velocity, projected along some direction, matches the phase velocity of a
fluid wave (linear oscillation). This can occur whenever grains stream faster
than any fluid wave. The wave itself can be quite general--sound waves,
magnetosonic waves, epicyclic oscillations, and Brunt-V\"ais\"al\"a
oscillations each generate instabilities, for example. We derive a simple
expression for the growth rates of these "resonant drag instabilities" (RDI).
This expression (i) illustrates why such instabilities are so virulent and
generic, and (ii) allows for simple analytic computation of RDI growth rates
and properties for different fluids. As examples, we introduce several new
instabilities, which could see application across a variety of physical systems
from atmospheres to protoplanetary disks, the interstellar medium, and galactic
outflows. The matrix-based resonance formalism we introduce can also be applied
more generally in other (nonfluid) contexts, providing a simple means for
calculating and understanding the stability properties of interacting systems.
