  Millisecond pulsars (MSPs) are short-period pulsars that are distinguished
from "normal" pulsars, not only by their short period, but also by their very
small spin-down rates and high probability of being in a binary system. These
properties are consistent with MSPs having a different evolutionary history to
normal pulsars, viz., neutron-star formation in an evolving binary system and
spin-up due to accretion from the binary companion. Their very stable periods
make MSPs nearly ideal probes of a wide variety of astrophysical phenomena. For
example, they have been used to detect planets around pulsars, to test the
accuracy of gravitational theories, to set limits on the low-frequency
gravitational-wave background in the Universe, and to establish pulsar-based
timescales that rival the best atomic-clock timescales in long-term stability.
MSPs also provide a window into stellar and binary evolution, often suggesting
exotic pathways to the observed systems. The X-ray accretion-powered MSPs, and
especially those that transition between an accreting X-ray MSP and a
non-accreting radio MSP, give important insight into the physics of accretion
on to highly magnetised neutron stars.
