CMOS-based microsensors combine, on a single chip, the necessary transducer elements and integrated circuits. This way, the microsensors benefit from well-established fabrication technologies and the possibility of on-chip circuitry. Besides sensor biasing and signal conditioning, added on-chip functionality, such as calibration, self-testing, and digital interfaces, can be implemented. A number of microsensors, including magnetic field and temperature sensors, are completely fabricated within the regular CMOS process sequence. A far larger number of microsystems can be realized by combining CMOS or BiCMOS technology with compatible micromachining and thin film deposition steps. These additional fabrication steps are performed either before, in-between, or after the regular CMOS process sequence. Commercially available examples include pressure sensors, accelerometers, gyroscopes, humidity sensors, mass flow sensors, and imaging devices. In the first part, the paper summarizes major technological approaches to CMOS-based sensors. In the second part, a packaged CMOS-based chemical microsystem, developed at ETH Zurich, Switzerland for the detection of volatile organic compounds in air is highlighted. On a single chip, the microsystem combines a sensor array featuring three different sensing principles with circuitry for sensor biasing, signal read-out, analog-to-digital conversion, and digital interfacing. The chemical microsystem is fabricated using an industrial CMOS technology in combination with post-processing bulk-micromachining to release the micromechanical sensor structures. After packaging the microsystem using flip-chip technology, the three sensor structures are coated with chemically sensitive polymer films. Absorption of volatile organic compounds in the polymer films results in a change of the (physical) film properties, such as the mass, dielectric constant, or temperature, which is then recorded by the underlying sensor structure.