A Review of Line and Continuum Correlations in AGNs
  This review addresses the current observational status of the Baldwin Effect (BE), an empirical anti-correlation between emission line equivalent width and source luminosity for active galactic nuclei. We provide a brief history of the BE, and discuss selection effects and analysis methods relevant to its study. Existing research points to a number of detailed patterns in the BE, involving emission-line profiles and ionization, luminosity range, and radio properties, that provide clues to the underlying physics driving the correlation. We present a comparative analysis of the "global" BE for AGN ensembles and the "intrinsic" BE seen in variable Seyfert nuclei, and also examine similarities and differences in BEs reported in the optical/UV and X-ray bandpasses. A summary of measurements for quasars at high redshift (z > 4) suggests that the BE persists among these extreme objects. We close with an assessment of current needs and future opportunities for strengthening the observational picture of the BE. 
Patrick S. Osmer (Ohio State U.) and Joseph C. Shields (Ohio U.)