ABSTRACT: We present local models of advection-dominated accretion flows (ADAF) into supermassive black holes. The thermal balance for ions and electrons is solved as a function of radius while using an approximation to the radial structure of the flow. Energy is exchanged between ions and electrons only by Coulomb collisions, and viscous heating is assumed to heat ions and electrons according to the mass ratio. We include thermal conductivity as a local cooling process for electrons and ions separately. The resulting synchrotron spectrum of thermal electrons is applied to Sgr A^*. We investigate the luminosity of ADAF's as a function of the mass accretion rate and show that the transition from advection-dominated flows to thin standard accretion disks can explain the distinction between low-luminosity AGN on the one hand and Quasars and Seyfert 1 nuclei on the other.