or Pontificium Institutum Studiorum Orientalium Ecclesiarum (PIO), a post-graduate Institute in Rome, founded by Benedict XV (pope, 1914-1922) by his Motu Proprio, Orientis catholici (15 October 1917), studies the dogmatic, liturgical, spiritual and canonical traditions of: 
- the Orthodox Churches accepting Chalcedon (451) and in communion with Constantinople, nowadays called "Eastern Orthodox"; 
- the non-Chalcedonian Armenian, Coptic, Ethiopian (including the Eritrean patriarchate set up in 1993), Syrian Orthodox and Malankara Orthodox Churches, together known as "Oriental Orthodox"; 
- the Church of the East, formerly described, incorrectly, as Nestorian; and 
- the 21 Eastern Catholic Churches, not all deriving from previously existing Orthodox Churches.

Courses at the PIO, open to all, started in October 1918. 

Benedict XV showed ecumenical foresight, since to this day many Churches study only their own tradition. As substitute secretary to Cardinal Mariano Rampolla, Secretary of State to Leo XIII (pope, 1878-1903), Giacomo della Chiesa (Benedict XV) witnessed the decline of the Ottoman Empire and wondered who would assume the "sick man of Europe’s" heritage.

This "Oriental question," at the basis of the Crimean War (1853-1856) and the Young Turks’ Revolution (1908), emerged at the Eucharistic Congress (Jerusalem 1893), under Cardinal legate Benoît-Marie Langénieux (+1904). 

The foundation of the “Congregation of the Oriental Church” (later: Churches) as independent from Propaganda Fide, by the Motu Proprio of 1 May 1917, Providentis Dei, anticipated the creation of the PIO by a few months. The Institute’s first locale was a hospice called dei Convertendi, or “Casa di Raffaello,” in Piazza Scossacavalli, later demolished to build Via della Conciliazione in front of St Peter’s Square.

At first the PIO seemed like a white elephant, with Roman authorities hesitating between the Institute as a training-school for missionaries and as a place for scientific research; eventually, it was this second option which prevailed. Pius XI (1922-1939) and Abbot (later Blessed) Ildefonse Schuster, OSB, preferred assigning the Institute to only one order, the Jesuits, to guarantee stability (1922), when, due to affinities and a common library, the Institute moved to the premisses of the Pontifical Biblical Institute in Piazza della Pilotta; and in 1926 to its present residence in Piazza S. Maria Maggiore, 7, Rome. As first Jesuit President—the title was later changed to Rector—was chosen Michel d’Herbigny (later bishop), whose book on V. Solov’ev had been crowned with a price of the French Academy; he was to remain in his post for ten years, but, in 1933, he started losing the Pope’s confidence and fell into complete disgrace (1937-1957). But by the time he left, the PIO was well established: Benedict XV gave the Institute the right to grant degrees (1920), in 1923 it started publishing Orientalia Christiana, which after the 100th number branched off, in 1935, into Orientalia Christiana Periodica for articles and reviews, and Orientalia Christiana Analecta for monographs. In 1992 a new publication which studies Eastern canon law, Kanonika, was started; Ivan ðuñek served as secretary to the commission for the codification of the Codex Canonum Ecclesiarum Orientalium.

Indeed, in 1971, a new faculty of Eastern canon law, the only one of its kind in the world, was set up, thanks to ðuñek’s insistence and Clement Pujol’s friendship with Cardinal Amintore Cicognani. The PIO thus has two faculties: Canon Law, and Ecclesiastical Sciences, including theology, patrology, spirituality, liturgy and history and languages. Programmes were streamlined according to the Vatican guidelines in Deus Scientiarum Dominus (1932) and in Divina Sapientia (1979). Jesuits had long studied the East and published patristic texts, Bollandists specialized in Eastern hagiography, too, others such as Antonio Possevino and Pjotr Skarga, engaged in polemics with the East. It was now time to harvest. Initially few were ready to publish syntheses, as Martin Jugie, AA, did a few years after leaving the PIO, but others, such as Irénée Hausherr, published on special themes.

World War II gave a lull, but the monumental Concilium Florentinum I-XI, (1940-1975), and the Anaphorae Syriacae, I-III, ed. Alphonse Raes, 1939-1981, were started. With Vatican II, the Institute became more overtly ecumenical (Georges Dejaifve, John Long, Hans-Peter Kolvenbach). In the aftermath of 1989 the Institute was flooded by students from the East. To face the emergency, the Cardinal Prefect of the Congregation of the Eastern Churches, in 1993, became also the Magnus Cancellarius of the PIO. In the wake of these events V.  Rev. H.-P. Kolvenbach, Superior General of the Jesuits, established in 1991 a new Research Centre affiliated academically to the PIO on the premisses of a palazzo in via Paolina 25 left to the Society of Jesus by Frau Anna Maria Grünhut Bartoletti Aletti, and named after her husband, Centro Ezio Aletti, with Marko Rupnik, as its first director. But by the end of the millennium a number of faculties had sprung up in the Eastern countries so that the number of students which were 375 in the academic year 1992-1993 and reached the climax in 1997-1998 with 435, returned to 374 for 2001-2002. In the meantime, new priorities have emerged such as the crisis in ecumenism with the Christian East and the recurrent problem of the Middle East. A theological synthesis has not yet been produced which would integrate the various dogmatic (Martin Jugie, Theophilus Spá…il, Mauricio Gordillo, Bernhard Schultze), spiritual (Irénée Hausherr, Ivan Kologrivof, Tomáš Cardinal Špidlík), liturgical (Juan Mateos, Jean-Michel Hanssens, Louis Ligier, Miguel Arranz, Robert Taft) and canonical aspects (Emil Herman, Ivan ðuñek, Clement Pujol, George Nedungatt) of the Eastern Churches, without ignoring the historical (Georg Hofmann, Carmelo Capizzi, Vincenzo Poggi), the archaeological (Guillaume Jerphanion, Vincenzo Ruggieri). Texts of Arab Christian Literature (Khalil Samir Khalil) and of the various Churches, e.g., Georgian (Michel van Esbroeck) as well as studies of the various Eastern areas (Placid Podipara for Malabar, Albert Ammann, Gustav Wetter, Gerhard Podskalsky, Constantine Simon and Gino Piovesana for Russia).

The Library of the Institute, with some 200,000 volumes, contains a unique collection of books on the Christian East, created by the generosity of Doña Victorina de Larrianga y Arriaga and Mgr. Jules Tiberghien, and the travels of Cyrille Korolevsky and Eugène (later Cardinal) Tisserant. Among non-Jesuit professors may be mentioned Mgr. Paul Mulla and Vartaped Levon Zekiyan; among former students, Patriarch Bartholomaeus and Alois Cardinal Grillmeier.

List of Presidents/Rectors: Antoine Delpuch (=vice-president), M.Afr.; Ildefonso Schuster, OSB; all Jesuits: Michel d’Herbigny, Emil Herman, Ignacio Ortiz de Urbina, Alphonse Raes, Joseph Gill, Ivan ðuñek, Georges Dejaifve, Eduard Huber, Peter-Hans Kolvenbach, Gilles Pelland, Gino Piovesana, Clarence Gallagher, again Gilles Pelland and Hèctor Vall. “Orientis catholici,” AAS 9 (1917) 531-532; “Quod nobis in condenda,” AAS 12 (1920) 440-441; E.G. Farrugia, SJ (ed.), The Pontifical Oriental Institute: The First Seventyfive Years, Rome 1993; V. Poggi, SJ, Per la storia del Pontificio Istituto Orientale: Saggi sull’istituzione, i suoi uomini e l’Oriente cristiano, Roma 2000; A. Tamborra, Chiesa cattolica e Ortodossia russa, Milano 1992; R.F. Taft & J.L. Dugan (ed.s), Il 75° Anniversario del Pontificio Istituto Orientale: Atti delle celebrazioni giubilari, 15-17 ottobre 1992, Roma 1994; G. Traina, “Institut Pontifical Oriental,” DHGE XXV, 1333-1336. E.G. Farrugia, SJ.
 
Among the outstanding names may be mentioned Guillaume de Jerphanion, Irénée Hausherr, Juan Mateos, Alfons Raes, Miguel Arranz, Kologrivov, Thomas Cardinal Spidlik, Robert Taft, G. Dejaifve, P.-H. Kolvenbach, E. Herman, I. Zuzek, G. Nedungatt, Hofmann, Krajcar, W. de Vries, G. Piovesana, Gustav Wetter, V. Poggi, J. Gill, C. Capizzi, J. Podipara, Paul Mulla, Samir, van Esbroeck.