Level monitoring instrumentation is an essential part of hydrocarbon processing facilities, and has, together with separator technology, been widely addressed over the last decade. Key issues are production capacity, product enhancement, and well-flow control. The reliability and accuracy of the level instrumentation, and its ability to monitor all the interface layers of the separator, including the thickness of the foam and the oil–water emulsion, are particularly important when considering the level instrumentation as the main sensing element in the automatic control of the separator vessel. Lately, industry focus has been placed on optimal automatic control to improve the quality of the production output, and to minimize the use of expensive and environmentally undesirable separation enhancing chemicals. Recent developments in hydrocarbon production include subsea separation stations, where the constraints placed on the reliability and accuracy of the level instrumentation are especially demanding. This paper presents level interface monitoring developments based on electrical, ultrasonic, thermal, and nucleonic physical principles for three-phase hydrocarbon separators, and introduces the notion of tomometry, meaning multipoint cross-sectional metering aiming to acquire information on the cross-sectional flow-component distribution in the process vessel intended for control purposes.