Solid　oxide　cells　(SOCs)　are　all　solid　ceramic　devices　with　the　dual　functionality　of　solid　oxide　fuel　cells　(SOFCs)　to　convert　the　chemical　energy　of　fuels　like　H-2,　natural　gas　and　other　hydrocarbons　to　electricity　and　of　solid　oxide　electrolysis　cells　(SOECs)　to　store　renewable　electric　energy　of　sun　and　wind　in　hydrogen　fuel.　Among　the　electrochemical　energy　conversion　and　storage　devices,　SOCs　are　the　most　clean　and　efficient　technology　with　unique　dual　functionality.　Due　to　the　high　operation　temperature　(typically　600-800　degrees　C),　SOCs　exhibit　many　advantages　over　other　energy　conversion　devices,　such　as　low　material　cost,　high　efficiency　and　fuel　flexibility.　There　has　been　rapid　development　of　SOC　technologies　over　the　last　decade　with　significant　advantages　and　progress　in　key　materials　and　a　fundamental　understanding　of　key　issues　such　as　an　electrode,　electrolyte,　performance　degradation,　poisoning,　and　stack　design.　The　reversible　polarization　also　has　a　critical　effect　on　the　surface　segregation　and　stability　of　the　electrode　and　electrode/electrolyte　interface.　This　critical　review　starts　with　a　brief　introduction,　working　principles　and　thermodynamics　of　SOC　technology　to　readers　with　interests　in　this　rapidly　developing　and　emerging　field.　Then　the　key　materials　currently　used　in　SOCs　are　summarized,　followed　by　the　discussion　of　the　most　advanced　electrode　modification　methods　and　critical　issues　of　SOCs,　including　the　surface　chemistry,　segregation,　electrode/electrolyte　interface　and　varying　material　degradation　mechanisms　under　reversible　operations.　The　challenges　and　prospects　of　SOC　technology　for　future　developments　are　discussed.