Development of a theory that pursuit to explain the nursing's nature, specific field and work methodology. It is founded on Maslow's theory to achieve that nursing is a service to Man with the object of assist him to get their basic needs and by this manner contribue to maintain him in equilibrium in time and space; to prevent the human lost of balance or, in such a case, to revert this lost in new states of equilibrium. From this theory results the concepts of nursing: to assist, assistance and care in nursing. Also some propositions and principles are proposed. The development of a methodology is based upon this Human Basic Needs Theory, related and consequently Nursing Process is based in six phases: Nursing History, Nursing Diagnosis, Assistencial plan, Nursing care plan, Evolution and Nursing Prognosis. It is emphasized the concerning of the development of skills named basic tools to be possible the execution of the Nursing Process.