Observational measures of AGNs allow us to directly infer some basic physical properties of quasars including (1) physical conditions and chemical abundances in the broad-line emitting gas as well as (2) radius of the line-emitting region, (3) black hole mass, (4) and Eddington ratio. In this chapter, we seek information on how these quantities are measured. One of the surprises of recent times, that would have shocked the discoverers in the 1960s are the large BH masses and high chemical abundances inferred for even some of the highest redshift quasars known.