ABSTRACT This chapter focuses on the development of sensors and sensor systems for harsh environments. Among various applications with high commercial impact, combustion optimisation and emission control in small-scale boilers is considered as an illustrative application which can benefit from the employment of a multimeasurand sensor system able to cope with harsh environment conditions. More specifically, silicon on insulator (SOI) is proposed as common technology platform for the realisation of a diode temperature sensor, a thermal flow sensor, a capacitive humidity sensor, a chemiresistive oxygen sensor, a diode ultraviolet (UV) photosensor, and a non-dispersive-infra-red (NDIR) carbon dioxide sensor. Considerations regarding circuitry, packaging and system integration and testing are also included, along with a detailed analysis of each single sensing device.