Fast ion conducting (FIC) sulfide glasses are ideal candidates for solid electrolytes used in Li battery applications because they have high ionic conductivity and may be tailored for extreme operating conditions through the addition of modifiers. An effort has been put forth to develop sulfide glass compositions possessing chemical stability necessary for production and thermal stability for a wide variety of applications while still retaining high ionic conductivity;A few new series of FIC glasses have been developed that have exceptional conductivities combined with high Tgs and good electrochemical stability. The structure of the glass network generally dictates the bulk properties of the glass, such as the ionic conductivity, density, thermal stability, and chemical stability. The structure of the glass network in the Li2S + GeS2 + Ga2S3 and Li 2S + GeS2 + La2S3 systems was performed using Raman and Infrared spectroscopy. The effects of concentration variations of each glass component along with the effects of additional glass modifiers such as LiI and BaS can be observed with the change in bulk properties, but can be explained using the structural analysis results obtained from the Raman and IR spectroscopy. The optimized glasses have room temperature conductivities of >10-3(O cm)-1 and Tgs in excess of 300°C. An increase in Ga2S3 concentration leads to the reduction of non-bridging sulfurs in the glass thus improving the thermal stability of the glass. The substitution of La2S 3 for Ga2S3 gives a slight improvement in the ionic conductivity and chemical stability of the glass. The addition of LiI is found to improve the glass formation and conductivity with only moderate decreases in the Tg (< 50°C) and the addition Of BaS improves the chemical stability of the glasses in dry air.