High temperature hermetic seal is essential for utilizing the full potentials of planar solid oxide fuel/electrolyzer cells. A seal glass needs to have excellent thermal and chemical stabilities, mechanical integrity, and sealing ability in stringent oxidizing and reducing environments and for hundreds of thermal cycles. Comprehensive analysis and understanding are needed in the design of a seal glass in order to meet the demanding requirements. In this review, seal requirements and the advantages of glass-based seals are first discussed. Different glass compositions are reviewed from thermal, chemical, mechanical, and electrical property point of view. Based on these considerations, glass composition design approaches are provided that aid in search of the best seal glass that can offer all the desired properties and stabilities. Required thermal properties such as thermal expansion coefficient, glass transition temperature, and softening temperature have been achieved in several alkaline earth borosilicate glass systems. Interfacial compatibility with other cell components has also been obtained for several alkaline earth borosilicate glass systems. However, long-term thermal and chemical stabilities are yet to be achieved. Among all the glass systems studied, a boron-free SrO-La2O3-Al2O3-SiO2 seal glass has been specifically discussed because it has met all the thermal and chemical properties along with high thermal and chemical stabilities. For future endeavors, the relationships between seal glass constituents, glass network structures, required thermal, chemical, mechanical, and electrical properties need to be established in order to improve sealing performance while maintaining design flexibility and low fabrication cost. © 2009 Elsevier B.V. All rights reserved.