The fact that many installations of a UPS (Uninterruptable Power Supply) work at all often appears to be more due to good fortune than good management. There are many considerations to the correct selection of a UPS about which many computer system owners or operaters may be unaware. The realisation of the need for a UPS usually follows some type of mains power disaster. While many system owners and operators may have experienced problems with mains power, they may not be aware of the full extent of these. This presentation will show the extent of power problems and present some of the main points that need to be considered to properly select a suitable UPS for a given situation. The intention of this presentation is not to provide a complete coverage of all technical deatils, but to enable an appreciation of the major points.