In the design of underground rock caverns, failure of both the ultimate and serviceability limit states need to be considered. Ultimate limit state refers to collapse, in which the stresses exceed the strength of the rock masses. Failure of the serviceability limit state refers to excessive deformations resulting in difficulties during excavation such as lining placement and reinforcement installation. In addition, it is widely recognized that a deterministic analysis of either the factor of safety or the calculated displacement gives only a partial representation of the true margin of safety/reliability, since the uncertainties in the design parameters affect the probability of failure. This chapter uses numerical modeling to assess both the ultimate and the serviceability limit states of underground single and twin rock caverns. The global factor of safety is used as the criterion for the ultimate limit state and the calculated percent strain around the cavern opening is adopted as the serviceability limit state criterion. Based on the numerical results, simple logarithmic regression models were developed for estimating the global factor of safety and the induced percent strain of the single and twin caverns, respectively. Simplified procedures are proposed for evaluating the failure probabilities for deep underground rock caverns for preliminary design applications.