This report is a user guide to geoscience data contained in exploration reports filed in the Geoscience Data Centre of the Ontario Geological Survey. The data cover over 10,000 geological, geophysical, or geochemical reports submitted by the mining industry since the mid-1940s to early 1984 in compliance with the Mining Act and exploration incentive programs. The guide explains the five ways used to identify exploration reports filed in the Centre, and provides information on the use of keywords and on the contents of report titles. Appendices include a list of abbreviations; a map index to National Topographic System quadrangles for Ontario; maps showing Mineral Exploration Assistance Program designated areas, regional & resident geologists' areas, locations of Mining Recorder offices and drill core libraries, and a graph showing the number of exploration reports by year.