Preface.- Understanding of semiconducting metal oxides gas sensors.- Semiconducting metal morphology and sensing performance.- Semiconducting metal composition and sensing performance.- Semiconducting metal microstructure and sensing performance.- Gas sensing performance and evaluation criteria.- New approach to better performance.- Sensing mechanism of semiconducting metal oxides gas sensors.- Sensing devices of semiconducting metal oxides.- Concerns in practical gas sensor fabrications.- Applications of semiconducting metal oxides gas sensors.- Summary and outlook.