Operational hearing protection and maintenance of audibility of signals and speech are considered force multipliers in military operations, increasing Soldier survivability and lethality. The in-field research described in this paper was conducted to examine operational performance effects of three different hearing enhancement protection systems (HEPS) that are intended to provide both protection and audibility. The experiment utilized operationally-defined measures in full-scale, simulated combat scenarios with Army ROTC Cadet Soldiers as subjects. The Soldiers' operational performance was evaluated in two missions: reconnaissance and raid (attack on enemy camp). Both missions had substantial hearing requirements, including communications, signal detection/recognition, and distance judgments. Operational performance was measured by objective metrics of Squad performance, including the distances required to detect an enemy insurgent camp under each HEPS, and by subjective metrics, such as the Army's dimensions of combat-related mission success as evaluated by Army Officers who served as training leaders/observers. Other subjective ratings were obtained after each training exercise from both the Officers and the Soldiers, including detailed impressions about each HEPS after extended use. Two of the three HEPS were electronic sound transmission devices (comprising an ambient sound pass-through filtering and amplification circuit): a Peltor Comtac II circumaural headset (NRR=21; 16 dB maximum gain); and a Communications Enhancement Protection System (CEPS) (NRR=29; 36 dB maximum gain). One passive, level-dependent HEPS was used, the yellow end of the Combat Arms Earplug, which provides amplitude-sensitive attenuation that sharply increases when the ambient sound is above about 110 dB (e.g., due to a gunshot), but which provides an NRR of 0 and very little attenuation below 1000 Hz in lower ambient noise levels. In the military mission entailing location of and attack on an enemy camp, the CEPS device resulted in the longest (earliest) average auditory detection distance of the camp (400 feet), followed by the Peltor (233 feet) and then the Combat Arms Earplug (150 feet), in comparison to detection by the unprotected, normal ear at about 220 feet. Commanding officers' ratings of mission performance and overall success slightly favored the electronic HEPS, but these ratings were dependent upon the particular mission undertaken. Ergonomics and usability issues abounded with the electronic HEPS, and the Soldiers' subjective ratings showed variability across all three devices, with device preference depending upon the particular dimension being rated (e.g., comfort vs. hearing ability). Clearly, the results of this in-field experiment demonstrate that more development is needed to achieve the levels of hearing performance and user acceptance from the HEPS that is desirable and needed for combat conditions. In this vein, it is important to note that HEPS designs are continually evolving, and certain upgrades to the three devices evaluated in the late 2006 timeframe of this study have occurred and further evaluations are thus warranted.