Canonization (or canonisation) is the act by which a Christian church declares a deceased person to be a saint, upon which declaration the person is included in the canon, or list, of recognized saints. Originally, individuals were recognized as saints without any formal process (as it is still done in the Orthodox Church). The process is most commonly used in, although not limited to, the Catholic Church.

The first persons the Christian Church honored as saints were the martyrs. Pious legends of their deaths were considered to affirm the truth of their faith in Christ, and formalization and celebration of these legends served to legitimize and propagate the doctrines of the Church and serve as examples.

The Latin Rite's Canon of the Mass contains the names only of martyrs, along with that of the Virgin Mary and, since 1962, that of Saint Joseph.

By the fourth century, however, "confessors"—people who had confessed their faith not by dying but by word and life—began to be venerated publicly. Examples of such people are Saint Hilarion and Saint Ephrem the Syrian in the East, and Saint Martin of Tours and Saint Hilary of Poitiers in the West. Their names were inserted in the diptychs, the lists of saints explicitly venerated in the liturgy, and their tombs were honoured like those of the martyrs. Since the witness of their lives was not as unequivocal as that of the martyrs, they were venerated publicly only with the approval by the local bishop.