Since the development of the liquid dominated geothermal reservoir at Wairakei, New Zealand in 1950s, various separator designs have been utilised to enable the separation of steam and water from two-phase geothermal fluid. This is to ensure that only dry and clean steam enters the turbine and generates electricity. Information from several existing geothermal fields shows that there are two common separator designs, the vertical cyclone separator and the horizontal separator. Both designs are reported to have high separation efficiency in the order of 99.9% or higher. The vertical cyclone separator is normally found at power stations with strong influence by the technology from New Zealand. The horizontal separator is normally found at power stations with strong influence by technology from Iceland, Japan, Russia and the US. The vertical cyclone designs are based on the experience in Wairakei and Kawerau in the 1950s and 1960s, and the modelling work by Lazalde-Crabtree's (1984). While the principles of the geothermal horizontal separator design were only reported by Gerunda (1981). This paper reviews the steam-water separator designs that are commonly used in geothermal steam fields worldwide. The general steps to design the separator for any given geothermal fluid are presented. This is starting from the selection of optimum separation pressure, predicting the separator efficiency and calculating the internal pressure drop. Recent research that utilises the numerical approach using Computational Fluid Dynamics (CFD) to obtain better understanding of the fluid behaviour within the separator is reported. Unpublished data from the early 1950s Wairakei trials are presented in this work. They show that the breakdown velocity increases with the reduction in the internal diameter of separator body. The measurement of the separator efficiency is also discussed. Practical design aspects for the optimum locating of the separator and the main separator design considerations are also given. Recent concepts in separator designs are also presented and discussed. (C) 2014 Elsevier Ltd. All rights reserved.