Oil reserves in 12 of California's 52 giant fields (fields with estimated recovery > 100 million barrels of oil) have continued to appreciate well past the age range at which most fields cease to show significant increases in ultimate recovery. Most of these fields were discovered between 1890 and 1920 and grew to volumes greater than 500 million barrels in their first two decades. Growth of reserves in these fields accelerated in th e1950s and 1960s and is mostly explained by application of secondary and tertiary recovery technicques, primarily waterflooding and thermal recovery. The remaining three-fourths of California's giant fields show a pattern of growth in which fields cease to grow significantly by 20-30 years following recovery. virtually all of these fields have estimated ultimate recoveries less than about 500 million barrels and most are in the 100-200 million barrel range. Three of six offshore giant fields, all discovered between 1966 and 1981, have shown decreases in their estimated ultimate sizes within about the first decade after production began, presumably because production volumes ailed to match initial projections. The data suggest that: 1. Only fields that attain an estimated ultimate size of several hundred million barrels shortly after discovery and have geologic characterisics that make them suceptible to advanced recovery techniques are likely to show substantial late growth. 2. Offshore fields are less likely to show significant growth, probably because projections based on modern seismic reflection and reservoir test data are unlikely to underestimate the volume of oil in the field. 3. Secondary and tertiary recovery programs rather than field extensions or new pool discoveries are responsible for most of the significant growth of reserves in California. 4. field size data collected ove rmany decades provide a more comprehensive context for inferring reasons for reserve appreciation than shorter data series such as the Oil and Gas Integrated Field file (OGIFF) from the U.S. Department of Energy's Energy Information Administration (EIA). 5. Efforts to project future growth in California fields, and perhaps fields in other regions, should focus on evaulating the potential for enhanced recovery in fields with current estimated ultimate recoveries of about 250-500 million barrels. 6. By analogy with oil, attempts to project growth in gas reservoirs, in California and perhaps elsewhere, should focus on larger fields with lower permeability reservoirs where advances in recovery technology, such as perhaps horizontal drilling, are more likely to add substantial reserves.