  Gravitational waves emitted by distorted black holes---such as those arising
from the coalescence of two neutron stars or black holes---carry not only
information about the corresponding spacetime but also about the underlying
theory of gravity. Although general relativity remains the simplest, most
elegant and viable theory of gravitation, there are generic and robust
arguments indicating that it is not the ultimate description of the
gravitational universe. Here we focus on a particularly appealing extension of
general relativity, which corrects Einstein's theory through the addition of
terms which are second order in curvature: the topological Gauss-Bonnet
invariant coupled to a dilaton. We study gravitational-wave emission from black
holes in this theory, and {\bf(i)} find strong evidence that black holes are
linearly (mode) stable against both axial and polar perturbations; {\bf(ii)}
discuss how the quasinormal modes of black holes can be excited during
collisions involving black holes, and finally {\bf(iii)} show that future
ringdown detections with large signal-to-noise ratio would improve current
constraints on the coupling parameter of the theory.
