Green, P. J.Silverman, J. D.Cameron, R. A.Kim, D.‐W.Wilkes, BelindaBarkhouse, W. A.LaCluyze, A.Morris, D.Mossman, AmyGhosh, H.Grimes, J. P.Jannuzi, B. T.Tananbaum, HarveyAldcroft, ThomasBaldwin, J. A.Chaffee, F. H.Dey, A.Dosaj, A.Evans, NancyFan, X.Foltz, C.Gaetz, TerranceHooper, E. J.Kashyap, VinayMathur, S.McGarry, M. B.Romero‐Colmenero, E.Smith, M. G.Smith, P. S.Smith, R. C.Torres, GuillermoViklinin, AlexeyWik, D. R.2017-02-132004Green, P. J., J. D. Silverman, R. A. Cameron, D.‐W. Kim, B. J. Wilkes, W. A. Barkhouse, A. LaCluyze, et al. 2004. “The Chandra Multiwavelength Project: Optical Follow‐up of Serendipitous Chandra Sources.” The Astrophysical Journal Supplement Series 150 (1) (January): 43–71. doi:10.1086/379818.0067-0049http://nrs.harvard.edu/urn-3:HUL.InstRepos:30212197We present follow-up optical g', r', and i' imaging and spectroscopy of serendipitous X-ray sources detected in six archival Chandra images included in the Chandra Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected between 3 × 10-16 and 2 × 10-13 (with a median flux of 3 × 10-15) ergs cm-2 s-1, we find optical counterparts for 377 (78%), or 335 (68%) counting only unique counterparts. We present spectroscopic classifications for 125 objects, representing 75% of sources with r* < 21 optical counterparts (63% to r* = 22). Of all classified objects, 63 (50%) are broad-line active galactic nuclei (AGNs), which tend to be blue in (g*-r*) colors. X-ray information efficiently segregates these quasars from stars, which otherwise strongly overlap in these SDSS colors until z > 3.5. We identify 28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%) are absorption line galaxies. Eight galaxies lacking broad-line emission have X-ray luminosities that require they host an AGN (logLX > 43). Half of these have hard X-ray emission suggesting that high gas columns obscure both the X-ray continuum and the broad emission line regions. We find objects in our sample that show signs of X-ray or optical absorption, or both, but with no strong evidence that these properties are coupled. ChaMP's deep X-ray and optical imaging enable multiband selection of small and/or high-redshift groups and clusters. In these six fields we have discovered three new clusters of galaxies, two with z > 0.4, and one with photometric evidence for a similar redshift.en-USGalaxies: activeQuasars: generalSurveysX-rays: galaxiesJournal Article2017-02-1310.1086/379818