Piarists, or Fathers (Regular Clerks) of the Pious Schools (scholarum iriarum, whence the popular name), a religious order in the Roman Catholic church, whose members take, in addition to the three common monastic vows, a fourth, to devote themselves to the gratuitous instruction of youth. The order was founded at Rome by St. Joseph Casalanza or Cala-sanctius (1556-1648), a Spanish priest of noble birth, who in 1597, in union with three other priests, opened a free school, which was soon attended by upward of 700 children. In 1617 Pope Paul V. conferred on the corporation of teachers the rank of a religious congregation, and in 1621 Gregory XV. gave them all the privileges of a religious order. The same pope in 1622 confirmed their rule, and appointed Oasalanza their first general. The order was suppressed by Innocent X., in consequence of serious internal dissensions, but reestablished by Clement IX. It spread rapidly through Italy, Germany, and Poland, and became eminently popular. The Piarists suffered less than any other order from the reformatory decrees of Joseph II. of Austria, and were exempted from the general suppression of convents in Spain in 1836. More recently they were equally favored in the Sardinian dominions.

In 1860 they possessed 28 houses in Italy, 33 in Germany, 32 in Hungary and its dependencies, 14 in Poland, and about 30 in Spain. They are now suppressed in Italy, like other religious orders. In .1870 they had 29 houses in Cisleithan Austria, about an equal number in Hungary, and a few in Poland and Spain. At the head of the order is a general, who is elected by the general chapter for six years, and resides together with a procurator general and two assistants at Rome. Every province is governed by a provincial, and every college has a rector and vice rector.