Voltage stabilizing circuits employing thermionic tubes can be classified into four groups, according to their derivation from (1) the transconductance bridge, (2) the amplification factor bridge, (3) the simple degenerative amplifier, and (4) combination circuits involving two or more of the foregoing classes or utilizing amplified control voltages. It is shown that the variational performance of a stabilizer is characterized by an internal output resistance and a stabilization ratio. The effect of source resistance on these performance parameters is discussed in the general case and expressions for these parameters are evaluated for ten stabilizer circuits, seven of which are original with the authors. All but two of the circuits can be adjusted to yield perfect stabilization and four have also a very low internal resistance. Glow discharge tubes are discussed as bias battery substitutes. A simple a.c. bridge technique which is convenient for the experimental determination of stabilizer performance under normal load conditions is described in an appendix.