The Ukrainian Greek Catholic Church (UGCC) is the largest Eastern Particular Church in the Catholic union of churches. The Primate of the Church holds the office of Archbishop-Major of Kyiv-Halych and All Rus, although the bishops and faithful of the church have acclaimed him a Patriarch and have requested the Patriarch of Rome to recognize his position.

The Christianity that UGCC embraces was first accepted by the Grand Prince Volodymyr the Great in 988. After the Great Schism of 1054, it was in union with Constantinople. Then, after the union of Brest-Litovsk in 1595, it entered again into union with the Patriarch of Rome.

In 1917, the area known as Rus became, temporarily, independent and known as Ukraine. Many people from this area had already immigrated to the United States and established churches. They were known as Russins. At the time of the brief independence of Ukraine, many    immigrants began calling themselves Ukrainians. Some objected and split the American Greek-Catholic Church into two separate entities: the Ukrainian Greek-Catholic Church and the American Byzantine-Catholic Church, also known as Ruthenians (a name given to the church from the Latin Church). In the U.S. there are approximately an equal number of bishops, priests and faithful in the two Churches. The Byzantine-Catholic Church is the only American sui juris (self-governing) Church. The Ukrainian Greek-Catholic Church and other U.S. Eastern Catholic Churches have Synods in their original countries.

The UGCC is now a world-wide Church and has some 40 hierarchs in over a dozen countries on four continents. It has three metropolitans outside Ukraine. They are in the U.S., Canada and Poland.

While within Ukraine itself the UGCC is a minority faith, being a distant second to the majority of three separate Ukrainian Orthodox Churches, it is the second largest religious organization in Ukraine in terms of number of religious communities. In terms of number of faithful, the Ukrainian Greek Catholic Church ranks third in allegiance among the population of Ukraine.

Currently the Ukrainian Greek-Catholic Church is predominate in three western oblasts of Ukraine, including about half the population of Lviv. It is, however, the largest of the Eastern Catholic Churches in the U.S. The U.S.  Province is composed of the Archeparchy of Philadelphia and the Eparchies of Chicago, Parma and Stamford.