  We present here a summary of the scientific goals behind the Gravity and
Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH)
observing program. The primary targets can be divided into two classes:
stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the
centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be
divided into various X-ray spectral states: thermal-dominant (disk), hard
(radio jet), and steep power-law (hot corona). These different spectral states
are thought to be generated by different accretion geometries and emission
mechanisms. X-ray polarization is an ideal tool for probing the geometry around
these BHs and revealing the specific properties of the accreting gas.
