  The last decade has been marked by great interest in the dynamics of vortices
moving at high ($>$10\,km/s) velocities in superconductors. However, the
flux-flow instability (FFI) prevents its exploration and sets practical limits
for the use of vortices in various applications. Even so, FFI has turned into a
valuable tool for studying the quasiparticle energy relaxation in
superconductors, with the view of enhancement of the single-photon detection
capability of micrometer-wide strips operated at large bias currents. In this
context, the failure of ``global'' FFI models for materials with strong
intrinsic and edge vortex pinning has urged the elaboration of ``local'' FFI
models. This chapter outlines the recent advances in the research on FFI and
highlights superconductors with perfect edges and weak volume pinning as
prospective materials for studying fast vortex dynamics and nonequilibrium
superconductivity. This manuscript is based on the author-prepared version of
the review published in Encyclopedia of Condensed Matter Physics, vol. 2 (2024)
735--754.
