Both eyes, although I don't know if that's the way we were taught because I have always shot with both eyes open. I am right handed but left eye dominate so using both eyes helps that. Never had an issues using both eyes to shoot with the aim point. They would all take a beating and I never saw one break that I can recall. The end of ours had a honeycomb insert so at to help keep light from hitting it and giving away our position. I don't know if the civilian version has those.