Sea mines have been important in naval warfare throughout history and continue to be so today. They have caused major damage to naval forces, slowed or stopped naval actions and commercial shipping, and forced the alteration of strategic and tactical plans. The threat posed by sea mines continues, and is increasing, in today’s world of inexpensive advanced electronics, nanotechnology, and multiple potential enemies, some of which are difficult to identify. This report assesses the Department of the Navy’s capabilities for conducting naval mining and countermining sea operations.