  Here we review empirical evidence for the possible existence of tachyons,
superluminal particles having m^2 < 0: The review considers searches for new
particles that might be tachyons, as well as evidence that neutrinos are
tachyons from data which may have been gathered for other purposes. Much of the
second half of the paper is devoted to the 3 + 3 neutrino model including a
tachyonic mass state, which has empirical support from a variety of areas.
Although this is primarily a review article, it contains several new results.
