Many shales previously thought of as only source rocks are now recognized as self-sourcing reservoirs that contain large volumes of natural gas and liquid hydrocarbons that can be produced by use of horizontal drilling and hydraulic fracturing. However, shale-gas resources and development economics are uncertain, and these uncertainties beg for a probabilistic solution. Our objective was to probabilistically determine the distribution of technically recoverable resources in highly uncertain and risky shale-gas reservoirs for seven world regions. To assess technically recoverable resources, we used the Unconventional Gas Resource Assessment System, which integrates Monte Carlo simulation with an analytical reservoir simulator, to derive a representative probability distribution of 25-year recovery factors (RFs) from five shale-gas plays in the US: the Barnett, Eagle Ford, Marcellus, Fayetteville, and Haynesville shales. The RFs for the five shale-gas plays follow a general beta-distribution with a mean value of 25%. Finally, we extended the distribution of RFs gained from the five shale-gas plays in the US to estimate technically recoverable shale-gas resources for the seven world regions. World technically recoverable shale-gas resources were estimated to range from 4,400 (P90) to 24,000 (P10) Tcf. This work provides important statistics for the five shale-gas plays in the US. Results of this work verify the existence of significant technically recoverable shale-gas resources and can help the industry better target its exploitation efforts in shale-gas plays worldwide.