See A. J. Fremantle, The Papal Encyclicals in Their Historical Context (1963).
Within Catholicism in recent times, an encyclical is generally used for significant issues, and is second in importance only to the highest ranking document now issued by popes, an Apostolic Constitution. However, the designation 'encyclical' does not always denote such a degree of significance. The archives at the Vatican website currently classify certain encyclicals as "Apostolic Exhortations". This informal term generally indicates documents with a broader audience than the bishops alone.
It is not to be thought that what is set down in Encyclical letters does not demand assent in itself, because in this the popes do not exercise the supreme power of their magisterium. For these matters are taught by the ordinary magisterium, regarding which the following is pertinent: “He who heareth you, heareth Me.” (Luke 10:16); and usually what is set forth and inculcated in Encyclical Letters, already pertains to Catholic doctrine. But if the Supreme Pontiffs in their acts, after due consideration, express an opinion on a hitherto controversial matter, it is clear to all that this matter, according to the mind and will of the same Pontiffs, cannot any longer be considered a question of free discussion among theologians.
- —Humani Generis
Encyclicals indicate high Papal priority for a issue at a given time. Pontiffs define when, and under which circumstances encyclicals should be issued. They may choose to issue an apostolic constitution, bull, encyclical, apostolic letter or give a papal speech. Popes have differed on the use of encyclicals: on the issue of birth control and contraception, Pope Pius XI issued the encyclical Casti Connubii, while Pope Pius XII gave a speech to midwives and the medical profession, clarifying his position on the issue. Pope Paul VI published an encyclical Humanae Vitae on the same topic. On matters of war and peace, Pope Pius XII issued ten encyclicals, mostly after 1945, three of them protesting the Soviet invasion of Hungary in order to crackdown on the Hungarian Revolution in 1956: Datis Nuperrime, Sertum Laetitiae, Luctuosissimi Eventus. Pope Paul VI spoke about the war in Vietnam and Pope John Paul II, issued a protest against the war in Iraq using the medium of speeches. On social issues, Pope Leo XIII promulgated Rerum Novarum (1891), which was followed by the Quadragesimo Anno (1931) of Pius XI, and the Centesimus Annus (1991) of John Paul II. Pius XII spoke on the same topic to a consistory of cardinals, in his Christmas messages and to numerous academic and professional associations.
*Pope Saint Pius V 7 January 1566 to 1 May 1572
*Pope Benedict XIV (1740-1758)
*Pope Blessed Pius IX (1846-1878)
*Pope Leo XIII (1878-1903)
* Pope Saint Pius X (1903-1914)
*Pope Benedict XV (1914-1922)
*Pope Pius XI (1922-1939)
*Pope Pius XII (1939-1958)
*Pope Blessed John XXIII (1958-1963)
*Pope Paul VI (1963-1978)
*Pope John Paul II (1978-2005)
*Pope Benedict XVI (2005 to present)