  I present a review by epoch of baryons in the intergalactic medium (IGM),
from the first star until today. Recent observations indicate a protracted
period of reionization, suggesting multiple populations of reionizers;
detection of these z>6 sources is a key goal that is now coming within reach.
The optical Lyman alpha forest (2<z<4) is well-described by the Fluctuating
Gunn-Peterson Approximation, but puzzling observations of associated metal
lines and nearby galaxies may provide insights into galactic feedback
processes. IGM studies at z<1.5 are progressing rapidly thanks to ultraviolet
absorption line studies of both Ly-alpha absorbers and the warm-hot
intergalactic medium. Cosmological hydrodynamic simulations have played an
integral part in these advances, and have helped to shape our understanding of
the IGM at each epoch. Working jointly, observations and theory continue to
expand our knowledge of the IGM as the earliest stages of galaxy formation and
the dominant reservoir of baryons at all redshifts.
