  We present 30 optical spectra and 49 photometric epochs sampling the first
517 days after discovery of supernova (SN) 1999em, and derive its distance
through the expanding photosphere method (EPM). SN 1999em is shown to be a Type
II-plateau (II-P) event, with a photometric plateau lasting until about 100
days after explosion. We identify the dominant ions responsible for most of the
absorption features seen in the optical portion of the spectrum during the
plateau phase. We find the distance to SN 1999em to be D = 8.2 +/- 0.6 Mpc,
with an explosion date of 5.3 +/- 1.4 days before discovery.
  We examine potential sources of systematic error in EPM-derived distances,
and find the most significant to result from uncertainty in the theoretical
modeling of the flux distribution emitted by the SN photosphere (i.e., the
``flux dilution factor''). We compare previously derived EPM distances to 5 SNe
II in galaxies for which a recently revised Cepheid distance exists from the
HST Key Project and find D(Cepheids) / D(EPM) = 0.96 +/- 0.09. Finally, we
investigate the possible use of SNe II-P as standard candles and find that for
8 photometrically confirmed SNe II-P with previously derived EPM distances and
SN 1999em, the mean plateau absolute brightness is M_V(plateau) = -16.4 +/- 0.6
mag, implying that distances good to ~30% (1-sigma) may be possible without the
need for a complete EPM analysis. At M_V(plateau) = -15.9 +/- 0.2 mag, SN
1999em is somewhat fainter than the average SN II-P. The general consistency of
absolute SNe II-P brightness during the plateau suggests that the standard
candle assumption may allow SNe II-P to be viable cosmological beacons at z >
2.
