http://www.formsofaddress.info/Doctorate.html... 1) Holders of doctorates who work in academia or research institutions are addressed as Dr. (Name) professionally and socially in a salutation or conversation. Thus a PhD professor at a college, a PhD in biology doing scientific research, and a PhD principal at an elementary school all use Dr. and everybody thinks it is normal. NOTE: At some universities it is traditional to address faculty holding of academic doctorates as Mr. (Name) or Professor (Name) and not to address as Dr. (Name). If you know it is the tradition it is correct to use it. But for those outside the academic community it is acceptable to address holders of doctorates as Dr. (Name) in writing or oral address... 3) Holders of doctorates who work outside academia or research don't always prefer to be addressed as Dr. (Name). in a salutation or conversation. In the USA "Dr." may be used depending on the work environment and/or when the degree isn't pertinent to the conversation. E.g., a PhD in finance working at a bank or a PhD in American history working in software development are not likely to insist on being addressed as Dr. (Name). But always ask for their preference. Use of, or omitting, the honorific can be a sensitive issue to some individuals! And, outside the USA I observe people holding a PhD do want to be addressed as Dr. (Name).

How to Address a Person with a PhD
How to Address a Person holding a Doctorate

     1) Holders of doctorates who work in academia or research institutions are addressed as Dr. (Name) professionally and socially in a salutation or conversation.  Thus a PhD professor at a college, a PhD in biology doing scientific research, and a PhD principal at an elementary school all use Dr. and everybody thinks it is normal.
     NOTE: At some universities it is traditional to address faculty holding of academic doctorates as Mr. (Name) or Professor (Name) and not to address as Dr. (Name).  If you know it is the tradition it is correct to use it. But for those outside the academic community it is acceptable to address holders of doctorates as Dr. (Name) in writing or oral address.

    2) Protestant clergy with doctorates are typically addressed as Dr. (Name) in a salutation or conversation too. I specify Protestant here because not all clergy is.  For example, neither priests (addressed as Father[Name]) nor rabbis (addressed as Rabbi [Name]) holding doctorates are ever addressed as Dr. [Name]. They stick with Father[Name]) and as Rabbi [Name].

    3) Holders of doctorates who work outside academia or research don't always prefer to be addressed as Dr. (Name). in a salutation or conversation.
     In the USA "Dr." may be used depending on the work environment and/or when the degree isn't pertinent to the conversation.  E.g., a PhD in finance working at a bank or a PhD in American history working in software development are not likely to insist on being addressed as Dr. (Name).   But always ask for their preference. Use of, or omitting, the honorific can be a sensitive issue to some individuals!
     And, outside the USA I observe people holding a PhD do want to be addressed as Dr. (Name).
    4) In hospitals and some other healthcare environments as well there is often a practice no one holding a doctoral degree except the physicians (medical doctors, dentists, osteopaths, podiatrists, veterinarians ... ) is addressed as Dr. (Name). This is out of consideration for the patients who want to know who are the doctors and who are nurses, support staff and allied professionals. It can be confusing with so many people walking around in white!
     This makes for some unhappy professionals who earned doctorates in hospital administration, pharmacy, physical therapy and nursing, etc. who might prefer to be addressed as Dr. (Name) too.  It's my understanding that all of these professionals might well be addressed as Dr. (Name) in other situations outside healthcare (teaching or consulting, for example). But for patients in the doctor's office, clinic, or hospital the practice of reserving Dr. (Name) for the physicians makes sense.

    5) All that said, ultimately how one is addressed by others is up to the individual and usually everyone goes along.  For example, if you and I meet a woman who identifies herself as Monsignor Alice ... I think it is unlikely she's a Roman Catholic Monsignor.  And, it's unusual she has only one name, like Pink, Rhianna, Sting, Cher, or Madonna. But we should directly address her in conversation as Monsignor Alice, it's nice to meet you  ... because that's what she says her name is.  But, when she's out of range, we will all be talking about her