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The enormous amounts of energy radiated from the active nuclei of galaxies vary on short time scales, and the emission regions are difficult to observe. To provide a complete understanding of these phenomena a wide variety of studies is presented in this volume. The contributions are broadly divided between line and continuum variability, with observational results, methodological approaches, and theoretical models accompanying each. The final part is devoted to the important aspect of propagation-induced variability.

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Have all questions been asked?.- Line and continuum variability in NGC 4151.- The optical, ultraviolet and X-ray variations in NGC 4151.- Mapping the BLR in NGC 5548 & NGC 4151.- Ground-based studies of emission-line variability: Recent results for NGC 5548 and future plans.- Variability of line profiles in NGC 5548.- Continuum variability in NGC 5548: Implications for theoretical models.- Variability in Markarian 279.- A search for variability in PKS 1302-102.- Meteorology of the broad line region.- An atlas of 2-dimensional transfer functions of the Broad Line Region.- Limits on the expansion of the radio source OQ 208 in Mkn 668.- BLR models and the L-M relation for AGNs.- Variability of BL Lac objects in the radio regime.- Intraday radio variability of quasars and BL Lac objects.- The high frequency properties of synchrotron radiation.- Synchrotron source models and the infrared-optical variability of blazars.- Rapid variability of BL Lac objects in the optical regime.- UV variability of blazars.- The connection between Frequency Dependent Polarization (FDP) and growing radio shocks in blazars.- Optical observations of rapid variability in the QSO PG 0117+213.- The connection between blazars and compact radio sources.- VLBI knots and superluminal motion.- Relativistic jet models and variable knot emission.- Particle acceleration and variability: Magnetic reconnection in AGN.- Electron-ion coupling in Compton-heated plasmas.- Variable radio sources and the two-fluid models.- Infrared variability of active nuclei.- Continuum variability in quasars and Seyfert galaxies.- The variability of Ly-alpha in 3C273.- The electromagnetic spectrum of the radio-quiet quasar 1821+643 and comparison with 3C 273.- Variability of active galactic nuclei: A theorist´s view.- The stability of thick disks.- Brightness and color variations of accretion disks: Implications for the parameters.- Structure and variability in broad absorption lines of quasar spectra.- Are the broad emission lines of quasars affected by gravitational microlensing?.- Gravitational microlensing and the Hamburg quasar monitoring program.- Lensing of BL Lac objects.

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A host of theoretical and observational research on the variability of active nuclei of galaxies is collected in this volume. It gives to the researcher a complete view of the present understanding of these phenomena.

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