Abstract : The present study developed casualty investigation procedures that focused on communications problems. These procedures were applied by U.S. Coast Guard (USCG) Investigating Officers in their investigation of 589 marine casualties over a seven-month period. Analysis of the resulting casualty reports determined that communications is a prevalent causal factor in marine casualties, being a factor in 18 percent of critical vessel casualties, 28 percent of critical personnel injuries, and contributing to 19 percent of critical marine casualties overall. The investigations procedures also identified characteristics and causes of communications problems. The single largest problem involved mariners who did not communicate when appropriate. Two types of faulty assumptions were usually the cause of this: they either misinterpreted the situation and did not perceive a threat, or they incorrectly assumed that others were aware of the problem and would take care of it. Training in developing team situation awareness is suggested to combat the first problem. Better crew resource management, specifically empowering crew members to speak up when a threat is perceived, would correct the second problem and potentially reduce communications-related casualties by 29 percent.