In contrast to remote instruments on orbiting spacecraft, in situ or "in place" instruments are needed for detailed chemical, biological and physical measurements that often require direct contact with planetary surface material. Many future experiments will require that the instrument execute complex sample processing such as extraction, concentration and labeling of a target analyte from a soil sample. All this must be accomplished while exposed to severe planetary environments such as extreme temperatures, pressures, and radiation. Future NASA missions to Mars will use in situ sensor systems to look for evidence of past or present life, to study surface and subsurface soil chemistry and mineralogy as well as atmospheric chemistry.