  A great deal of emphasis on timing in the RXTE era has been on pushing toward
higher and higher frequency phenomena, particularly kHz QPOs. However, the
large areas of the RXTE pointed instruments provide another capability which is
key for the understanding of accreting X-ray pulsars -- the ability to
accumulate high quality spectra in a limited observing time. For the accreting
X-ray pulsars, with their relatively modest spin frequencies, this translates
into an ability to study broad band spectra as a function of pulse phase. This
is a critical tool, as pulsar spectra are strong functions of the geometry of
the "accretion mound" and the observers' viewing angle to the ~10^12 G magnetic
field. In particular, the appearance of "cyclotron lines" is sensitively
dependent on the viewing geometry, which must change with the rotation of the
star. These spectral features, seen in only a handful of objects, are quite
important, as they give us our only direct measure of neutron star magnetic
fields. Furthermore, they carry a great deal of information as to the geometry
and physical conditions in the accretion mound. In this paper, we review the
status of cyclotron line studies with the RXTE. We present an overview of
phase-averaged results and give examples of observations which illustrate the
power of phase-resolved spectroscopy.
