I review how AGN black hole masses are calculated from emission-line reverberation-mapping data, with particular attention to both assumptions and caveats. I discuss the empirical relationship between AGN luminosity and broad-line region radius that underpins the indirect methods by which most AGN masses are estimated. I also discuss how line widths are characterized in this method and illustrate how different ways of measuring the line-widths can lead to systematic errors in the mass scale. I discuss specific implications for NLS1 galaxies and consider whether the NLS1 phenomenon is better explained by source inclination or by Eddington rate, and conclude that there is evidence that both of these effects are contributing factors and that at least the high-Eddington rate NLS1s are physically similar to some high-luminosity quasars.
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