As the primary risk associated with deep-well injection is that isolation from the accessible environment might fail, this probability must be examined before drawing any conclusions regarding health or environmental risks from such a release. This chapter examines the risk from such a failure of isolation and provides an objective and quantitative analysis of the risk of waste isolation loss from Class I Hazardous (Class IH) underground injection wells that will allow meaningful identification and comparison of waste isolation subsystems as contributors to that risk. Because of the conservative assumptions used for failure event probabilities and the explicit treatment given to uncertainties in this analysis, it is believed that the risk of loss of waste isolation from Class IH wells is less than 10⁻⁶. Numerous conservative assumptions were used in this probabilistic risk assessment (PRA) that, combined with the explicit treatment of uncertainties (i.e., the Monte Carlo analysis), lend confidence to the conclusions of low risk. Because the failure location and timing of the individual events are critical to the development of these release scenarios, uncertainty would be reduced and knowledge improved if this information were collected and included in the databases maintained on Class I well failures. It is recommended that future assessments of the potential environmental risks associated with deep-well injection explicitly take into account the probability of release and the amount of waste that could be released by the mechanisms of feasible system failure scenarios.