Electrical heaters are seeing increased use in downhole applications. Low power heaters are used for viscosity flow assurance, paraffin buildup reduction and hydrate elimination. Electrical heating can be used where steam is not technically useful such as through permafrost, formations that are highly fractured with connectivity where SAGD is precluded and very deep wells with high pressure. Higher power electrical heating is now being use in formation stimulation for reduction of viscosity oil formations worldwide. The highest temperature applications are in the "Insitu Conversion Process" pioneered by Shell in the oil shale of Colorado. Polymer insulated heaters are used for temperatures under 400 °F and inorganic insulated heaters such as mineral insulated (MI) cable for temperatures as high as 1800 °F. The history of electrical heating applications and specific pilot results are reviewed. The paper reviews the technical characteristics of power source and temperature source electrical heaters. The paper reviews the history of electrical heater use in downhole applications, discusses the chemical, mechanical, and electrical challenges for downhole use and predicts the increased application of electrical heaters in the future. In South America there is a need for heaters as long as 7000 feet, operating at 4160 Volts with output of up to 350 watts per foot. The near well bore constraints of thermal diffusivity requiring a temperature source heater rather than a power source heater is discussed for use in Colorado Oil Shale applications. Process insitu time periods and energy balance for both conversion from Kerogen to bitumen; then upgrading the bitumen and to oil as light as 40 °API is also discussed. The time required for simple viscosity reduction heavy oil with suggested heater well spacing is reviewed.