Emission-line variability in Seyfert galaxies

Observations and theory of emission-line variability in Seyfert galaxies are reviewed in the context of standard photoionization models of the emission-line regions. Expected time scales for variability and systematic effects which can give misleading results are discussed. Published observations of emission-line variability are summarized. The implications of these observations are noted, particularly the physical consequences of broad-line regions that variability analysis indicates are an order of magnitude smaller than predicted by standard photoionization models.