Pope Benedict XVI has urged the cardinals responsible for nominating candidates for sainthood to be strict in their selection.
Pope Benedict XVI started John Paul II on the path to sainthood
The Pope's predecessor, John Paul II, created 482 saints - more than had been canonised in the last 500 years.
John Paul himself has been fast-tracked by the Vatican towards sainthood - a process that once took many years.
But Pope Benedict wants more caution to be exercised in the examination of miracles and martyrdom.
Proof of a miracle is usually required before a virtuous person can be made a saint.
In a letter to the Congregation for the Causes of Saints, the Pope emphasised the need to study "deeply" the subject of miracles, especially "in the light of the tradition of the Church, of modern theology, and of the most accredited discoveries of science".
"It should also be clearly borne in mind that unbroken Church practice establishes the need for a physical miracle, a moral miracle is not enough," he said.
Another road to sainthood is martyrdom, but the Pope also stressed the need for strict criteria.
"It is of course necessary to find incontrovertible proof of willingness to suffer martyrdom... and of the victim's acceptance thereof," he wrote.
But he added that the persecutor's hatred of the faith should also be apparent.
"If this element is lacking, there is no real martyrdom in accordance with the perennial theological and juridical doctrine of the Church," he said.