||Unconventional petroleum resources are naturallyoccurring deposits of liquid and solid hydrocarbons that have not been fully exploited due to unfavorable economics or inadequate technology. The term unconventional conveniently describes these deposits without imposing specific limits, such as density, viscosity, depth, or rock type. Most unconventional petroleum resources fall into the ultraheavy oil or tar sands category; some, such as the diatomaceous oil shales, do not. Heavy crude oils are generally in the 10° to 20° API gravity range, whereas unconventional (ultraheavy) oils are normally below 10° API. Although heavy crude oils are not the subject of this paper, it should be pointed out that they are, perhaps, the most significant petroleum resource in the state. Estimates of the original oil in place in heavy crude oil reservoirs in California range from 35 to 50 billion barrels. During the last 15 years, many of these heavy oil reservoirs have been actively exploited using thermal enhanced recovery techniques, resulting in approximately one billion barrels of additional oil recovery. Currently, California produces over 500,000 barrels per day of heavy crude oil and approximately half, or 250,000 barrels per day, is a direct result of thermal enhanced recovery. The distribution, physical characteristics, and potential of some of the more significant unconventional deposits in California are described in this paper. Although an in-depth discussion and assessment of all unconventional petroleum resources is not included, it is hoped this paper will stimulate investigation and lead to a complete study and quantitative analysis of such resources.