The aim of this paper is to provide a comprehensive review of the current measurement technologies available for on-line monitoring of the performance of the gas/oil/water separators, which are commonly used as the first stage of crude oil processing. Firstly, a short overview is given of the fluid physics governing the separation processes and the design of separation vessels. Simple theoretical models of separation physics are briefly reviewed and the process challenges outlined. This is followed by the description of existing measurement systems, those entering the market and likely future developments. The measurement systems are broadly divided into three categories: (1) those based on the assumption of a complete phase separation within the vessel; (2) those based on the concept of an array of sensors distributed across the heterogeneous layers, hence not requiring any assumptions regarding the actual phase distribution; and (3) more complex systems enabling cross-sectional imaging in the industrial process tomography sense.