Pioneering work in fitting the spectrum of the X-ray continuum to measure spin was carried out by Zhang et al. (1997a), and the method was advanced further by Gierlinski et al. (2001). Very recently, two developments have allowed this method to be applied more widely and with some confidence, namely: (1) models of thin accretion disks are now publicly available in XSPEC (“KERRBB”, Li et al. 2005; “KY”, Dovciak et al. 2004) that include all relativistic effects and allow one to fit for ; and (2) sophisticated disk atmosphere models now exist for computing the spectral hardening factor, $f_{col} = T_{col}/T_{eff}$ as a function of the Eddington-scaled luminosity of the disk (Davis et al. 2005).