This work, appearing on the 25th anniversary of "Humanae Vitae," describes the contents of the encyclical on family planning, summarizes reactions to it from western church leaders, and reflects on its relevance to modern societies. "Humanae Vitae" begins with a statement acknowledging the problems posed by population growth to societies and to individual families. Deteriorating conditions of housing and employment and increasing economic and educational inadequacies often make it difficult to raise large families in the modern world. Views of the role of women in society and of the value of marital love and the significance of the sex act in relation to marital love have undergone change. The document raised the question of whether it is time to entrust to couples greater responsibility in regulating their fertility. The document referred to the reasons why the Pope and the Church have the authority to answer such a question and described the care with which the document was prepared before moving to the second major part of the article, that concerning doctrinal principles. After remarking on the respect due to nature, to the purposes of the conjugal act which were identified as union and procreation, and to faithfulness to God's design, the Pope moved on to condemn means of birth regulation termed illicit, which include abortion and direct sterilization as well as contraception. 2 exceptions were recognized, therapeutic sterilization and recourse to infertile periods for couples who for serious reasons wished to avoid pregnancy. The third section instructed priests, couples, bishops, and others in their duties in relation to the contents of Humanae Vitae. The encyclical was praised by some church leaders but also questioned. The fact that the work was not presented as infallible and possible conflicts with the duty of men affirmed in Vatican II not to act against their consciences were noted. When Humanae Vitae was published, the world population was 3.6 billion. By 1992 it was 5.479 billion. Since the encyclical was not presented as infallible, an opening presumably exist to adapt its message to current realities.