Underground coal gasification (UCG) is a technology for exploiting deep coal resources. A major consideration in the design and operation of UCG is the selection of the oxidant and gasifying medium. Air, enriched-air, steam/ oxygen, water/oxygen and CO2/oxygen are potential oxidants for use in UCG. This article provides guidelines for the selection of the oxidant and gasifying medium through a combination of qualitative assessment and the use of a simplified cost model. It is found that air blown operations are popular for demonstration purposes and are favoured for coals at shallow to moderate depths and when project scale is small. Steam/oxygen blown gasification is suitable for coals at shallow to moderate depth and may be favoured when steam can be generated from waste heat sources in an integrated plant. Water/oxygen blown gasification is attractive when operating pressures are high (e.g. in deep coals) and when project scales are large. There is currently limited practical experience with carbon dioxide/oxygen blown gasification in underground coal gasification. (C) 2017 Elsevier B.V. All rights reserved.