Erected ca. 1075, it was subordinate to the Metropolitan Archdiocese of Gniezno, and as of 1818 to the Archdiocese of Warsaw. The last territorial changes were introduced by Pope John Paul II on 25 March 1992 by the bull Totus Tuus Poloniae populus. The capital of the diocese is Płock with the Cathedral of the Assumption of the Blessed Virgin Mary, Our Lady of Mazovia. The patrons of the diocese are St. Stanislaus, bishop and martyr, and St. Stanislaus Kostka, religious.