Super-massive binary black holes and emission lines in active galactic
  nuclei
  The broad emission spectral lines emitted from AGNs are our main probe of the
geometry and physics of the broad line region (BLR) close to the SMBH. There is
a group of AGNs that emits very broad and complex line profiles, showing two
displaced peaks, one blueshifted and one redshifted from the systemic velocity
defined by the narrow lines, or a single such peak. It has been proposed that
such line shapes could indicate a supermassive binary black hole (SMB) system.
We discuss here how the presence of an SMB will affect the BLRs of AGNs and
what the observational consequences might be.
  We review previous claims of SMBs based on broad line profiles and find that
they may have non-SMB explanations as a consequence of a complex BLR structure.
Because of these effects it is very hard to put limits on the number of SMBs
from broad line profiles. It is still possible, however, that unusual broad
line profiles in combination with other observational effects (line ratios,
quasi-periodical oscillations, spectropolarimetry, etc.) could be used for SMBs
detection.
  Some narrow lines (e.g., [O\,III]) in some AGNs show a double-peaked profile.
Such profiles can be caused by streams in the Narrow Line Region (NLR), but may
also indicate the presence of a kilo-parsec scale mergers. A few objects
indicated as double-peaked narrow line emitters are confirmed as kpc-scale
margers, but double-peaked narrow line profiles are mostly caused by the
complex NLR geometry.
  We briefly discuss the expected line profile of broad Fe K$\alpha$ that
probably originated in the accretion disk(s) around SMBs.
  Finally we consider rare configurations where a SMB system might be
gravitationally lensed by a foreground galaxy, and discuss the expected line
profiles in these systems.
