This chapter will review several solid state chemical sensors with focus on the importance of ultra high vacuum, UHV, for the development of this area. Examples of sensors will be given where processing of sensors and sensing layers as well as characterization of chemical sensors takes place in UHV as well as examples of sensors for operation in UHV. Applications of chemical sensors both already commercialized and still on the research level will be given. Sensor technologies will span from metal oxide sensors, field effect transistor sensors to surface plasmon resonance, SPR, sensors and microcalorimeters. Examples of new challenging novel sensor approaches like sensors based on indirect SPR sensing and ultra sensitive graphene-based sensors for NO₂ detection will also be given.