The New Testament and earliest Christian writings describe communities led by a plurality of presbyters, elders, working alongside a bishop, a collegial pattern only gradually giving way, over the second and third centuries, to the more familiar structure in which priests exercise sacramental ministry chiefly by delegation from, and in a single diocese's dependence on, one bishop who alone had originally presided at the Eucharist himself in the earliest, smaller Christian communities. As dioceses grew far too large for a bishop to preside at every celebration personally, priests took on the eucharistic and pastoral leadership of individual parishes in the bishop's place, the arrangement still governing parish life today.
The number of Catholic priests worldwide has declined significantly in several regions, particularly Western Europe and North America, since the mid twentieth century, even as the global Catholic population has grown, producing a widening priest-to-parishioner ratio that has prompted parish mergers, the increased use of lay ecclesial ministers for pastoral tasks not requiring ordination, and, especially in mission territories and parts of Africa and Asia, a notable flow of vocations in the opposite direction from where the shortage is most acute.