TL;DR: “Dr.” if they have a Ph.D. or other research/medical doctorate, “Professor” otherwise.

The Basics

This post is primarily intended for first-generation or international students who may not have had this previously explicitly explained. These rules are part of the “hidden curriculum” of academia, and it is my hope that this post helps make these rules a bit less hidden.

The default title for a professor (including Assistant, Associate, and Full Professors, Lecturers, Instructors, and Clinical Professors) is “Professor”. However, in the United States, the title of “Doctor” (abbreviated “Dr.”) is used for persons holding doctoral degrees such as Ph.D., M.D., D.O., D.M.A., etc., and this generally supersedes other titles. You don’t need a doctorate to teach at the college level in the U.S. - a master’s degree usually suffices, so many of your instructors may not have doctorates.

Other professional doctorates have the word “doctor” in the name of the degree, but do not confer the title, e.g., the J.D. degree. In law and other professional fields, it is even more likely that your professor will have their field’s qualification(s) instead of a doctorate.

A person should be addressed by their title and last name, e.g., “Dr. Takeshita” or “Professor Takeshita”. Hold off on less formal methods of address (e.g., “Hi Jon”, “Hey professor”) until you know the person well enough.

Outside the United States

These rules are flipped in most countries outside the United States, most particularly in most Asian countries. Elsewhere, a Ph.D. is often required to teach at the college level, so the title of “Professor” is considered to be higher than that of “Dr.”. Also, the custom in some other countries is to address professors by their first name, e.g., “Dr. Jonathan”, but this could be considered too familiar in the United States.

Signature

In your email signature, it’s best to use postnominals to identify yourself. This makes your educational qualifications clear without any ambiguity. If you’re still a student, then write “Ph.D. Student”. Don’t call yourself a Ph.D. Candidate until and unless you pass your university’s candidacy exam - the Ph.D. differs from other degrees in that candidacy must be earned, and is not given simply by matriculation.

Footnote: Medicine

The title of “Dr.” in a medical context is understood to refer to a physician. It is unethical and in some cases illegal for non-physicians to use the title of “Dr.” in a medical context. For students earning a Ph.D. in computer science (or any other discipline), be mindful to not overstep!