The last decade has witnessed an explosion in research on the leaching of solids associated with shale oil production. Federal and state agencies and industry have sponsored millions of dollars in research in this area at private and public institutions. The leaching properties of many samples of raw and spent shale's of widely differing histories have been examined using techniques ranging form simple batch tests, to exhaustive soxhlet extractions, to long-term field studies. This has produced, not surprisingly, a diversity of opinions regarding the composition of leachates and their environmental significance.