How to address the Catholic Clergy

Identify the clergyman's rank: Pope, Bishop, Priest, or Deacon.
Be aware of additional titles like "Doctor" or honorary titles, like "Monsignor."
Identify the form of address to be verbal or written.
Address Deacons verbally as "Mr. Last Name". If you are addressing a letter to a transitional deacon, use "Rev. Mr. Full Name". A transitional deacon is one who will be ordained a priest. Permanent deacons are addressed as "Mr. Last Name". Sometimes, a Deacon can be addressed, simply: "Deacon Last Name."
Address Western Church Priests verbally as "Father First Name". Some persons who are not Catholic and also certain Catholics prefer to address Catholic priests as "Reverend". In some parts of Europe,for example, in Italy, it is common even for Catholics to address a priest as "Reverendo" especially in the absence of any other title. "Reverend", while not originally a title, has become a title with usage and has been common in the U.S.A. for at least 40 years. It is only necessary to say the last name once, if at all, depending on the circumstances. After that, simply call him "Father". In writing a letter, address the letter to "The Reverend Full Name" and append any doctoral degree (like Ph.D. or D.D.) if the priest has such a degree. Do not address letters to priests as "Reverend Father". In the address field write "The Reverend Full Name, Doctoral Degrees only," if any. Begin your letter with "Dear Reverend Last Name" or "Reverend Last Name" for a greater degree of formality; or "Dear Father Last Name." (NB: This advice differs from that offered by Debretts, who suggest that a formal salutation is "Dear Reverend Father". It is incorrect to write Dear Reverend Smith, as "reverend" is an adjective and should be prefixed by "The" when using the priest's name; as you would do in the address, i.e. "The Reverend John Smith" and not Reverend John Smith. Indeed, on that point the preceding advice is contradictory.)
Address Bishops verbally as "Bishop Last Name". It is optional to add his last name. If you say the Bishop's last name, do so only once. After that, simply address him as "Bishop". Address letters to "The Most Reverend Full Name, D.D.", then add any other doctoral degrees, if he has any. In conversation, a bishop may be addressed "Bishop" without name. It is preferable to use no name than to call the Bishop by his first name. Simply address him as "Bishop". In some cases, the most formal way to address a Bishop is "Your Excellency". The equivalent for Archbishops is "Your Grace". However, many senior clergy find this form of address embarrassing, and are extremely unlikely to insist upon it.
Address Cardinals verbally as "Cardinal Last Name" or "Your Eminence" without name. If you address the Cardinal as "Cardinal Last Name", do so only once. After that, address him as "Your Eminence". Address letters to "His Eminence, First Name Cardinal Last Name." If the Cardinal is a Bishop, add "D.D." and any other doctoral degrees he may have, after his last name.
Address the Pope verbally as "Holy Father" or "Your Holiness". Never say the Pope's name in a conversation with him. Address letters to "His Holiness Pope Benedict XVI."