The Cardinal Deacons are the lowest-ranked of the three orders of Cardinals of the Roman Catholic Church. They derive originally from supervisors of the Church's works in various districts of Rome.

Until 1918 it was possible to become a Cardinal without entering Holy Orders, but only the order of Cardinal Deacons was open to those who were not priests. To become a Cardinal Priest one had to be a priest and to become a Cardinal Bishop one had to be a bishop. After 1918 it was established that all cardinals, even the Cardinal Deacons, had to be priests, and since 1962 all cardinals have been bishops with rare exceptions where permission was granted to decline episcopal consecration because of advanced age (for example, Avery Cardinal Dulles).

Under the 1587 decree of Pope Sixtus V that fixed the maximum size of the College of Cardinals until 1958, there were fourteen diaconates, but the number has increased. As of 2005 there were over fifty recognized titular diaconates, though only thirty cardinals were of the order of Deacons. Cardinal Deacons have long enjoyed the right to "opt for the order of Cardinal Priests" (optazione) after they have been Cardinal Deacons for ten years, and after this they rank in precedence as if they had been Cardinal Priests from when they first became Cardinals. They may on such elevation take a vacant title (church allotted as the titular dignity of a Cardinal Priest) or their existing diaconate may be elevated to title for that occasion.

Today, appointment as a Cardinal Deacon is usually granted to officials of the Roman Curia and to the aforementioned cardinals who have not been consecrated bishops. Bishops with pastoral responsibilities on the other hand are created Cardinal Priests.