  We present evidence for widespread relativistic effects in the central
regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies
observed by \asca, 14 show an iron K$\alpha$ line which is is resolved, with
mean width $\sigma_{\rm K\alpha}=0.43\pm 0.12$~keV for a gaussian profile (Full
Width at Half Maximum, FWHM$\sim 50,000$~km s$^{-1}$). However, many of the
line profiles are asymmetric. A strong red wing is indicative of gravitational
redshifts close to a central black hole and accretion disk models provide an
excellent description of the data. The peak energy of the line is 6.4~keV,
indicating that it arises by fluorescence in near-neutral material. Our fits
imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean
of 30\deg, consistent with orientation-dependent unification schemes.
Differences in the line profiles from source-to-source imply slight variations
in geometry, which cannot be accounted for solely by inclination. In most
cases, we require that the line emission arises from a range of radii. Although
a small contribution to the emission from a region other than the disk is not
ruled out, it is not generally required and has little effect on our
conclusions regarding the disk line. Our data are fit equally well with
rotating (Kerr) and non-rotating (Schwarzschild) black hole models. We find a
mean spectral index in the 3-10 keV range of $<\Gamma_{3-10}>=1.91\pm 0.07$
after accounting for the effects of reflection. Such observations probe the
innermost regions of AGN, and arguably provide the best evidence yet obtained
for the existence of super-massive black holes in the centers of active
galaxies.
