Many of these claim succession from the see of Utrecht, or from Eastern Orthodox or Eastern-Rite Catholic churches; others from Roman Catholic bishops that have ordained their own bishops after disputes with the Vatican.
According to Roman Catholicism, the ordination of a bishop is valid, even if illegal, if the bishop is in a line of succession of bishops dating back to the Apostles and the rites of ordination are properly performed (see Apostolic succession). That is why Catholics recognize the validity of ordinations in Eastern Orthodox and some other churches, but do not recognize Protestant ordinations.
Many people have claimed ordinations as bishops where it is questionable whether the ordination ever took place.