Orthodox
Does the Eucharistic bread need to be unleavened (azymes)?
The Latin Church uses unleavened bread following the Last Supper's likely Passover setting, while the East uses leavened bread emphasizing the risen, living Christ; the Catholic Church recognizes both as valid matter for a licit Eucharist.
Catholic teaching
The Church teaches that what matters for validity is wheat bread offered with the proper form and intention; the choice of leavened or unleavened bread is a matter of legitimate liturgical diversity between the Latin and Eastern traditions, not a doctrinal dispute about the Real Presence itself.
Biblical foundation
The Synoptic Gospels place the Last Supper within the Passover meal, when only unleavened bread was used (Matt 26:17-26), which the Latin tradition follows literally, while the Eastern tradition emphasizes the new life of the Resurrection symbolized by leaven, both rooted in the same Gospel event.
Scripture references
Historical evidence
The controversy over azymes flared sharply only around the schism of 1054, when it was used polemically on both sides, even though both usages had coexisted peacefully as regional customs for centuries before.
The Church Fathers
“This sacrament can be performed both in unleavened and in leavened bread.”