Pius IX (1846-1878) The longest serving pope since St. Peter, Pius IX convened the First Vatican Council, established the teaching of papal infallibility and used said infallibility to define the dogma of the Immaculate Conception of Mary (a belief that had been part of Catholic tradition for centuries). The reign of Pius IX also saw the loss of the Papal States, the territories directly under the rule of the pope. This began a period in which the pope became known as the "prisoner of the Vatican." He was also the first pope to be photographed. Pius IX was beatified by none other thath Pope John Paul II in 2000.
Leo XIII (1878-1903) The third longest serving pope since Peter, Leo XIII is probably best known as the father of modern Catholic social teaching, launched with the epic encyclical letter Rerum Novarum, which addressed the plight of workers and society in the context of the Industrial Revolution. The encyclical remains the social encyclical against which others are measures, with subsequent popes observing its anniversaries with encyclicals of their own, assessing the ongoing state of socioeconomic justice in the world. Leo XIII was the first pope to be filmed, reportedly blessing the camera afterward, and first to be audio recorded.
St. Pius X (1903-1914) The last pope to date to be canonized a sant, Pius X has been nicknamed "the pope of frequent communion" for encouraging Catholics to do just that -- receive the Eucharist regularly. He's also credited with lowering the age of First Communion to "the age of reason." Today's Catholics take both of these realities for granted.
Benedict XV (1914-1922) Pope during World War I, Benedict XV is remembered primarily as a "prophet of peace," something that led Joseph Ratzinger to take the name Benedict himself upon his 2005 election to the papacy. Along with his peace advocacy and humanitarian efforts in the face of the devastation of global war, Benedict XV also found time to promulgate the first ever Code of Canon Law in 1917. It's amazing the Church didn't get around to doing this till 20 centuries into its existence.
Pius XI (1922-1939) The pontificate of Pius XI is perhaps most notable for his signing of the Lateran Treaty, establishing the sovereign Vatican City state, as it is known today (kind of a big deal). Pius XI also inaugurated Vatican Radio, whose operations continue to present day. In the way of social teaching, Pius XI issued the 1931 encyclical Quadragesimo Anno to mark the 40th anniversary of Rerum Novarum. In his encyclical, Pius XI looked at the rebuilding of society amid the Great Depression.
Pius XII (1939-1958) Like Benedict XV, Pius XII had the challenge of leading the Church through a world war. In the midst of this, he had a reputation as a deeply prayerful man and versatile teacher, authoring many encyclicals. He also relaxed the restrictions for fasting before Communion. Perhaps most notably, Pius XII defined the dogma of the Assumption of Mary in 1954, making him the last pope to date to draw on papal infallibility.
John XXIII (1958-1963) Elected at age 76 and expected to be a mere transitional pope, the pontificate of John XXIII is better described as pivotal. Convening the Second Vatican Council, he ushered in a paradigm shift in Roman Catholicism, the most significant since the Reformation. Other Christians went from being "heretics and schismatics" to "our separated brethren." In the opening address of the Council, Pope John said the Church now prefers "the medicine of mercy" over a "spirit of condemnation." The pope himself embodied this change, traveling outside of the Vatican, even traveling to Assisi by train, and appealing to people around the world with his warmth and good humor. His groundbreaking encyclical on world peace, Pacem in Terris, was the first addressed not only to the bishops of the Church, but to all people of good will. He was beatified by John Paul II in 2000.
Paul VI (1963-1978) The pontificate of Paul VI was dominated by the completion and implementation of the Second Vatican Council, which saw sweeping reforms in liturgy, interreligious affairs and virtually every area of Catholic life. Pope Paul wrote several encyclicals, including affirmations of Catholic teaching on celibacy and birth control, and his own contribution to the body of Catholic social teaching with 1967's Populorum Progressio. In his day, Paul VI was considered the "pilgrim pope" given that he was the first pope in a long time to travel outside of Italy. His message even traveled beyond planet Earth with his blessing of Apollo 11.
John Paul I (Aug.-Sept. 1978) The proof that God doesn't waste popes might just lie with the 34-day pontificate of John Paul I. In under a month, John Paul I took two unprecedented steps. First: taking the first double name in the history of the papacy. Second: by refusing to be crowned, opting instead for a simple installation Mass. His successors to date have followed his lead on the this last point, a symbolic trajectory change at the very least.
John Paul II (1978-2005) The first non-Italian pope in 400 years and the first Polish pope ever, John Paul II outdid seemingly every papal record in his 26 years on the chair of Peter. He brought the Gospel and the papacy to nearly every corner of the globe in his expansive world travels. He wrote 14 encyclicals on topics including bioethics, Mary, the Eucharist, truth, the relationship between faith and reason, and the role of the papacy. He also oversaw the publication of the revised Code of Canon Law in 1983 and the Catechism of the Catholic Church. The illness of his later years provided a powerful witness to the dignity of suffering.
Benedict XVI (2005-present) Now coming up on six years into his pontificate, Benedict XVI has engaged in unprecedented levels of dialogue with the Muslim world, been an outspoken advocate on environmental issues, made his own contributions to Catholic social teaching, allowed wider use of the 1962 Latin Mass, and created ordinariates for traditional Anglicans to come into communion with Rome while retaining the unique character of their worship. And as one Vatican official recently noted, this is only the beginning of his pontificate ...