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ABSTRACT. 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, particularly at optical and ultraviolet wavelengths. In radio-loud AGN, bipolar jets emanating from the nucleus emit radio through gamma-ray 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 general properties of AGN. We summarize the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming. We outline the two most plausible unified schemes for radio-loud AGN, one linking the high-luminosity sources (quasars and luminous radio galaxies) and one the low-luminosity sources (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 and conclude they probably are powered by similar physical processes, at least within the relativistic jet. 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.

KEY WORDS: Active galaxies : radio-loud - Unification - BL Lac objects - Quasars

Table of Contents

1. INTRODUCTION
2. OBSERVED PROPERTIES AND EMPIRICAL CLASSIFICATION OF AGN
3. ANISOTROPIC RADIATION FROM OBSCURATION

   3.1. Polarimetric Evidence for Hidden Nuclear Regions

   3.2. Infrared and X-Ray Evidence for Hidden Nuclear Regions

   3.3. Anisotropic Illumination of Narrow Emission Line Gas

4. ANISOTROPIC RADIATION FROM RELATIVISTIC BEAMING

   4.1. Evidence for Relativistically Beamed Gamma-Rays

   4.2. Superluminal Motion in Radio Jets

   4.3. Observed Radio Jet Asymmetries

   4.4. Brightness Temperature Calculations and SSC Models

5. BASIS FOR UNIFICATION SCHEMES

   5.1. History of Radio-Loud Unification Schemes

   5.2. Distinction between FR I and FR II Radio Galaxies

   5.3. Isotropic Properties of Quasars and FR II Galaxies

   5.3.1. Extended Radio Emission

   5.3.2. Narrow Emission Lines

   5.3.3. Infrared Properties

   5.3.4. Host Galaxies

   5.3.5. Environments

   5.3.6. Cosmic Evolution

   5.4 Isotropic Properties of BL Lac Objects and FR I Galaxies

   5.4.1. Extended Radio Emission

   5.4.2. Narrow Emission Lines

   5.4.3. Host Galaxies

   5.4.4. Environments

   5.5. The Myth of Unbiased Selection

   5.6. Effect of Relativistic Beaming on Number Statistics

6. STATISTICAL UNIFICATION OF RADIO-LOUD AGN

   6.1. Unification of Radio Quasars and FR II Galaxies

   6.1.1. Content of the 2 Jy Sample of Radio Sources

   6.1.2. Observed LFs of High-Luminosity Radio Sources

   6.1.3. Beamed LFs of High-Luminosity Radio Sources

   6.2. Unification of BL Lac Objects and FR I Galaxies

   6.2.1. X-Ray and Radio Samples of BL Lac Objects

   6.2.2. Properties of X-Ray-Selected and Radio-Selected BL Lac Objects

   6.2.3. Population Statistics for X-Ray Samples

   6.2.4. Population Statistics for Radio Samples

   6.2.5. Relation of X-Ray- and Radio-Selected BL Lac Objects

   6.2.6. New Terminology and a New Connection Between BL Lac Classes

   6.3. Independent Estimates of Relativistic Beaming Parameters

7. RELATION OF QUASARS AND BL LAC OBJECTS

   7.1. Low- and High-Redshift BL Lac Objects

   7.2. Possible Connections between BL Lac Objects and FSRQ

   7.2.1. The Evolutionary Connection

   7.2.2. Multiwavelength Spectral Continuity of BL Lac Objects and FSRQ

   7.2.3. BL Lac Objects as Gravitationally Micro-Lensed FSRQ

8. THE VIABILITY OF UNIFIED SCHEMES

   8.1. Potential Problems with Unification

   8.1.1. Linear Sizes of Blazars and Radio Galaxies

   8.1.2. Dependence of Quasar Fraction on Redshift

   8.1.3. Absence of Superluminal Motion in Radio Galaxies

   8.1.4. The Parent Population of BL Lac Objects

   8.2. Possible Complications for Unification

   8.2.1. Properties of the Obscuring Torus

   8.2.2. Cosmic Evolution of Radio-Loud AGN

   8.2.3. Parameterization of Relativistic Beaming

   8.2.4. Compact Steep-Spectrum and Gigahertz Peaked-Spectrum Sources

   8.2.5. Selection Effects in the Identification of Quasars

   8.2.6. Extended Continuum Emission in Type 2 AGN

9. THE TEN MOST IMPORTANT QUESTIONS

1. RELATIVISTIC BEAMING PARAMETERS
2. DOPPLER ENHANCEMENT
3. RATIO OF CORE- TO EXTENDED-FLUX
4. GLOSSARY OF ACRONYMS
5. REFERENCES