Transformers are vital and high cost components of the electric power system. On-line monitoring is a tool that can assess the condition of these valuable assets in real time. The importance of a monitored transformer and the economic consequences of a failure, provide the basis for selecting a monitor with its potential cost benefits. The most basic and important transformer monitoring equipment is the on-line Dissolved Gas Analysis (DGA) monitor. This paper discusses the importance of on-line DGA monitoring for power transformers, types of available sensors and various qualities to examine when choosing a monitoring device.