In the early Church, those who presided over local Christian communities or Churches were called bishops. The term comes from the Greek word 'episcopos'. In the Greek context, the term was the name given to a civil officer who was the administrator of a subject state. The term was adopted by the Christians for the one who was overseer or president of the local Christian community. We see thus that the episcopos or bishop was necessarily linked to the local community or territory over which he presided and he had to reside among the people he served. He was thus essentially a 'residential bishop'.

This set up of each bishop presiding over a given territory or diocese was upset particularly with the rise of Islam in the 7th century. Christianity which had its birth in a South-West Asian country initially spread mainly in South-West Asia (Palestine, Syria, Asia Minor) and North Africa. The aggressive expansion of Islam brought these countries into the Islamic fold, and they have remained staunch adherents of Islam to this day.

The early Christian communities of these countries were thus swallowed up by Islam. Only the names of the ancient sees remain. When the sees came under Islamic domination, their bishops, who could no longer serve the people, went elsewhere and gave assistance to bishops of other dioceses, hoping however to return some day to their own dioceses. With the Moorish conquest of Southern Spain, a similar situation prevailed there too.

The displaced bishops who worked elsewhere were no longer residential bishops but only titular bishops. They bore the title of their diocese though not presiding over them. Many bishops never had the opportunity of returning to their dioceses. They remained titular bishops to the end of their days.

But the Church did not forget the venerable ancient sees. From the pontificate of Pope Leo X (1513-1521) started the custom of giving bishops who had no charge of dioceses the titles of ancient sees, such as Taumaca, Mulsuce, Olympus, Tipasa. These bishops thus became Titular Bishops. They were expected to inform themselves of the sees the title of which they bore.

Thus from a historical fact of there being at one time de facto titular bishops who had lost their sees, arose the practice of bishops, who do not have dioceses, being nominated to now-lost former sees over which they will not have any jurisdiction. The creation of titular bishops serves as a link between the present and the Church's historic past. Titular bishops are known as bishops 'in partibus infidelium', that is bishops in infidel territories, though they had once been Christian.

Today, there are several categories of bishops, engaged in work other than that of administering a diocese of their own, each of whom had been given the title of an ancient see. They are therefore Titular Bishops.

(a) Vicar Apostolic, a bishop administering generally an ecclesiastically undeveloped territory which is not a diocese and which he governs, not as its bishop, but in the name of the Supreme Pontiff.

(b) Auxiliary Bishop, a bishop who functions as assistant to a diocesan bishop.

(c) Coadjutor Bishop, same as an Auxiliary Bishop, but has the right of succession.

(d) Papal Legates (Nuncios, Pro-Nuncios, Internuncios, Apostolic Delegates).

(e) Bishops holding posts in the Curia, that is, bishops engage in the administrative work of the Church.