Petition for opening of Cause for Beatification was granted by the Apostolic Administrator of Manfredonia in November 0f 1969.

What is beatification?

A process resulting in the declaration by the Pope that a person of saintly life or heroic death may be called "Blessed" and considered to be in Heaven. The preliminary process is long and painstaking, with examinations into the life, reputation and works of the candidate to see whether he or she practiced heroic virtue, or died a heroic death for the Faith. Miracles must be proved to have resulted from prayers to the servant of God in the case of a confessor of the Faith, but not necessarily in case of a Martyr. Miracles are needed in both cases for canonization. [MCD]

In Padre Pio's case only one miracle was required. This is the practice by the Vatican for some time now, since the pontificate of John Paul II.

Some background on canonization in general:

Saint Ulrich of Augsberg, who died in 973 was the first official canonization by a pope. In the early days of the Church the Saints were Martyrs, known to have died for the faith.
The Church in these early days was small by modern comparison and the Saints honored were exclusively Martyrs who were publicly sentenced to death by the judges because they refused to deny the faith. This is why the canon or list of Saints is called the Martyrology.
 Those from the Old Testament who had died in the state of grace and those who had died in the state of grace before the Sacrament of Baptism in the New Testament  were awaiting the Resurrection of Christ, are also Saints, but did not need to go through the canonization process. Not every known "Martyr" necessarily dies for the faith, or there exists a question as such in some cases; so today Martyrs are also canonized after examination. But what about Saints who were not Martyrs?

In the first several hundred years of the Church there had not as yet been established a formal certification process for declaring someone a Saint. Generally the holiness of the person, his or her heroic virtues and the outpouring of public acclaim by those who had personal knowledge of the person sufficed. Eventually the bishops of each locality began to investigate to be certain that the public acclaim was valid, and not arising from mere popular sentiment among those who did not necessarily know the person, except by reputation. The bishop of a diocese possessed the right to authorize that religious honor be paid to the Saints within the limits of the diocese committed to his care. And it often happened that the faithful began and continued to pay homage to a Saint without any intervention of episcopal authority. Nor was there much danger of abuse arising from this unauthorized veneration, because it originated with those who knew him or her best. As it goes with human nature, most of us are more harshly judged by those who do know us best because they have to live with us, so to speak.

Yet, the Church was not unaware to the danger of error and abuse. To her is entrusted the custody of the deposit of faith, and, since God desires us to honor the Saints, He grants the Church the right to ascertain their sanctity. As early as the fourth century we find an example of the Church in such capacity. St. Optatus, in his history of the Donatist heresy, relates that at Carthage a Christian matron, named Lucilla, incurred the censures of the Church for having kissed the relics of a reputed Martyr whose claims to Martyrdom had not been juridically proved. In times of persecution, when imaginations are stirred and hearts are on fire, the Church has need of firmness to guard the purity of her liturgy and public worship against intemperate piety and zeal. The phrase "Vindicare martyrem" comes into current use at this period, and means that religious honor was not to be rendered to a reputed Martyr unless it was declared legitimate by the Church.

The history of canonization  reveals one certainty---the diligence and prudence exercised by the Church in testing the holiness of the Saints and regulating the cultus or devotion to be paid to them. St. Cyprian recommended that the utmost care be observed in investigating the claims of those who are said to have died for the faith. All the circumstances accompanying the Martyrdom were to be inquired into; the faith of those who suffered, and the motives that animated them were to be rigorously examined. His letters make it clear that the utmost caution was employed to prevent the recognition of undeserving persons. He died in 258. So you see, even then it was not a "free for all."

Sometimes a close scrutiny stalls indefinitely the cause of a known holy person because of odd anomalies. This happened with Thomas a Kempis, the priest and author of THE IMITATION OF CHRIST. He had been buried while still alive by accident---medical certainty of death not being as accurate as today. When they exhumed his body they discovered that some of his hair was clutched by his hands. Church authorities did not know what to make of this since he had not been laid to rest with his head in his hands. Some though that he may have awakened and died in despair. No one could decide. So nothing was ever done because the Church needs certainty. Now all those who testified on behalf of the cause of a Kempis told of his virtues and holiness. But for this strange discovery perhaps he would be known as St. Thomas a Kempis.

Other times the investigation of an otherwise holy person reveals one of the virtues is lacking. The cause is discontinued. For example, there was a priest, thought to be holy whose cause was being investigated. The Church learned that this priest had set a large sum of money aside to assist in the promotion of his saintly aspiration. In other words, he thought he would be proposed for sainthood and planned accordingly. Obviously, while he may had other good qualities, he had no humility, the most important virtue, the very foundation for all the other virtues. This does not mean he went to Hell, only that the Church cannot propose such a person for the honors of the altar unless humility can be proven, among other virtues. That priest could still have been in Purgatory for all she knew. Until this fact came to light all the people who admired him and were hoping for his canonization had not a hint. He seemed humble to them, perhaps a kind of natural modesty that can mimic humility.

Each diocese has examiners or have them supplied by the Church. Only until they decide there is sufficient grounds to proceed further, that a priest is appointed "Postulator" and the candidate is given the title "Servant of God." The Posulator acts under the direct supervision of the Holy See, specifically the Congregation for the Causes of Saints. He reports all information and findings to it. Then a commission of cardinals review the heroic virtues and his life. If the proposed Saint is deemed worthy, he then receives the title Venerable. This simply means the candidate possessed all the heroic virtues and lived a holy life, worthy of a Saint.

To be declared Beatified, one miracle is required [it used be two].

The required miracle must be verified through canonical investigation, following a procedure like that for heroic virtues. This one too is concluded  with a decree. Once the two decrees are promulgated, the first being that of Venerable, the Holy Father decides on beatification, which is the concession of public veneration, limited to a particular set of devotions in some locales, generally. The title of a beatified person is Blessed.

For canonization another miracle is needed, attributed to the intercession of the Blessed and having occurred after his beatification. The methods for proving an affirmed miracle are the same as those for beatification. Canonization is understood as the concession of public veneration in the Universal Church. A declaration that someone is a Saint is an act of infallibility.

In 1973 the Archbishop of Manfredonia consigned to Holy See all documents required for formal introduction of Cause for Beatification and Canonization of the Servant of God, Padre Pio of Pietrelcina, January 16.

On December 18, 1997, Pope John Paul II proclaims Padre Pio "Venerable."

Then a year later, on December 21, 1998 ,the Decree of the Congregation for the Causes of Saints is proclaimed which concerns the miracle attributed to the intercession of the Venerable Servant of God, Padre Pio of Pietrelcina.

And in 1999 on May 2, Ven. Pio of Pietrelcina is proclaimed "Blessed" by His Holiness, John Paul II, in Saint Peter's Square.

He was Canonized on June 16, 2002.