The series of papers, 'Nuclear Notes', prepared by the US Army Nuclear Agency is intended to clarify and explain various aspects of nuclear weapons phenomenology and usage. These papers are prepared in as nontechnical a fashion as the subject matter permits. They are oriented toward an audience assumed to be responsible for teaching or in some way evaluating the tactics and techniques of employing nuclear weapons in a conflict situation. Their dissemination will hopefully provide to the US Army accurate, up-to-date information of critical importance to a reasoned understanding of nuclear weapons on the battlefield. In this note, a description of the sources of blackout is provided; blackout interference to Army tactical communications systems is addressed; and several blackout problems that may be encountered in a theater nuclear war are discussed. Actions that may be taken to minimize blackout interference are also set forth.