Despite nearly universal agreement that hearing is critical to the success of military operations, very little quantitative data exists to support this assertion. Unfortunately, hearing-related issues abound across our military services. To design and implement effective measures to mitigate these issues, data is needed to determine the extent to which military effectiveness is impaired when speech communication falls below some specific measurable threshold. In this study, U.S. Navy personnel participated in two experiments to examine the impact of speech intelligibility on operational performance in an Aegis Combat System Command Information Center simulation. Subjects wore headsets with custom-designed software to control speech intelligibility in real time. In the first experiment, speech intelligibility was measured using the modified rhyme test (MRT). In the second experiment, subjects acted as either commanding officer or tactical action officer in a combat scenario divided into time segments, each conducted at a different level of speech intelligibility. Results indicate that mission success, as measured by the percentage of tasks accomplished, decreased dramatically for MRT scores below approximately 65 percent.