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We have compiled a large sample of low-redshift active galactic nuclei (AGN)identified via their emission line characteristics from the spectroscopic dataof the Sloan Digital Sky Survey. Since emission lines are often contaminated bystellar absorption lines, we developed an objective and efficient method ofsubtracting the stellar continuum from every galaxy spectrum before makingemission line measurements. The distribution of the measured H$\alpha$ FullWidth at Half Maxima values of emission line galaxies is strongly bimodal, withtwo populations separated at about 1,200km s$^{-1}$. This feature provides anatural separation between narrow-line and broad-line AGN. The narrow-line AGNare identified using standard emission line ratio diagnostic diagrams. 1,317broad-line and 3,074 narrow-line AGN are identified from about 100,000 galaxyspectra selected over 1151 square degrees. This sample is used in a companionpaper to determine the emission-line luminosity function of AGN.

Active Galactic Nuclei in the Sloan Digital Sky Survey. I. Sample Selection