  The appearance of active galactic nuclei (AGN) depends so strongly on
orientation that our current classification schemes are dominated by random
pointing directions instead of more interesting physical properties. Light from
the centers of many AGN is obscured by optically thick circumnuclear matter and
in radio-loud AGN, bipolar jets emanating from the nucleus emit light that is
relativistically beamed along the jet axes. Understanding the origin and
magnitude of radiation anisotropies in AGN allows us to unify different classes
of AGN; that is, to identify each single, underlying AGN type that gives rise
to different classes through different orientations.
  This review describes the unification of radio-loud AGN, which include radio
galaxies, quasars, and blazars. We describe the classification and properties
of AGN and summarize the evidence for anisotropic emission. We outline the two
most plausible unified schemes for radio-loud AGN, one linking quasars and
luminous radio galaxies and another linking BL~Lac objects and less luminous
radio galaxies. Using the formalism appropriate to samples biased by
relativistic beaming, we show the population statistics for two schemes are in
accordance with available data. We analyze the possible connections between
low- and high-luminosity radio-loud AGN. We review potential difficulties with
unification and conclude that none currently constitutes a serious problem. We
discuss likely complications to unified schemes that are suggested by realistic
physical considerations; these will be important to consider when more
comprehensive data for larger complete samples become available. We conclude
with a list of the ten questions we believe are the most pressing in this
field.
