Abstract This chapter addresses both the challenges and benefits of fabricating gas sensors using complementary metal oxide semiconductor (CMOS) technology and the integration of associated circuitry onto a single silicon chip. The main objective described here is, specifically, the development of ultra-low power (∼mW) resistive and calorimetric gas sensors that can operate at high temperatures (up to 600 °C) on a thin (∼μm) CMOS platform. Compatibility with standard CMOS technology permits high volumes of production (>100k dies per annum) at very low unit cost (<$1). We present, in turn, the design of interface electronics for these high-temperature gas sensors, the necessity of having sensor arrays when employing nonselective gas sensing materials, and the exploitation of a new generation of gas-sensitive nanomaterials for superior CMOS devices. Finally, the future prospects of smart CMOS-based gas (or odor) sensors are considered with particular focus on emerging applications and possible new markets.