By what title should I greet assistant and associate professors in emails and letters? I study in the UK and for post-doctorands up to senior lectures one uses "Dear Dr. X", but for professors one uses "Dear Professor. X".
It's interesting actually,
As an Australian undergrad I've never referred to any of my lecturers or tutors as anything but their first names, and would find an insistence on titles in such a situation to be... well, frankly an exercise in serious anal retention.
However while applying for postgraduate studies, all of my 'reach out' emails trying to attract supervisory interest use the appropriate titles. While almost all of the replies come back establishing a first-name basis for future correspondence, I wouldn't dream of initiating said correspondence informally.
Highlights cultural differences across countries too - I'm fairly certain that if a senior academic at my uni in my field started insisting that his or her students use their full titles, the rest of the department would give them a far less flattering unofficial one.
It's quite important to consider what your position is in this relationship, and you don't specify. In almost any situation, it is probably best to address your first email using the "Dr." or "Prof." title, to be polite, and to continue to do so until it seems that the tone of the responses are less formal. Of course, that is a subjective thing to judge. But I can't give you much advice about that!
You will have to read from the tone of the responses at what point (possibly after only one email) the Prof. is OK with a less formal form of address. If you are an undergrad student in their class, it might not hurt to stay with the more formal title, even if you are of a similar age to them.
I was trained in the UK and now work in the US system, and I feel like there is a lot of similarity. Basically, it's fairly informal, within limits. As an assistant professor myself, I do prefer undergrads to address me as "Dr." or "Prof." (as a US class instructor, not the same as the UK Professor) in more formal correspondence, and I think that's usual in the US and UK. Graduate students and above can normally expect to be safe to be on first name terms provided their emails remain fairly professional and carefully written, i.e. they don't rapidly degenerate into txtspeak or very casual, potentially rude, references or phrasing.
So, if you are a graduate student or post-doc, and you are at the same department, or even institution, I encourage you to consider using their first name once that initial correspondence has been exchanged and if they sign with their first name. It's never "wrong" to continue to use a more formal term of address, but it can sometimes become awkwardly too formal for relaxed academics.
Now, take into account the personality of your correspondent. If he or she is very senior in age or accomplishment, or maybe has given an impression of great self-importance (i.e., ego!) then you may be better off staying slightly more formal in case you are perceived to be disrespecting them before you know them more personally. Honestly, though, I think that is an increasingly rare (but not unheard of) phenomenon.
This is an old question but I feel that the answers were incomplete in that the question has so far been interpreted mostly along the lines of "how formally should I address someone". However, you can also read it as a question of "is an associate professor properly addressed as 'dr' or 'prof'"?
In some countries, for example, here in the Netherlands, somebody who is called an "assistent professor" or "associate professor" is not considered a (full) professor. For example, these researchers are not addressed like a professor in thesis defense ceremonies, they do not wear the same formal attire during official ceremonies etc. We definitely do not use the term "professor" for everybody who teaches in class. Adressing an assistent/associate professor as "professor" would thus not be appropriate here. (Professors are senior researchers who are typically the chair of a group of researchers and addressed as such. Becoming a full professor involves a university ceremony etc.). If you address a Dutch assistent or associate professor for the first time, I would use dr.+surname (typically, people become assistent or associate professor after obtaining a PhD, and you can check on the university website if in doubt whether they have a PhD). I would not use the job title ("Dear assistent professor X") in the same way that I would not address other people with their job function either ("Dear department manager Smith" ?!), but in some of the other responses this is suggested, so this might differ as well between countries.
Last note: Some people who have a PhD, do not like to be addressed as Mr/Ms X instead of dr.
Professor or Dr.
I would definitely not go with first name, many (including myself) would find that too familiar from someone they don't know - and some may take offense. You can't make any assumptions about this, so it's better to err on the side of more formal until/unless you find out otherwise (or are invited to address the person by their first name).
Exeter University has published a protocol for how to address Associate Professors in various contexts. What's important about that is not so much the content of the protocol, as the fact that Exeter University believes it's up to the individual university to decide, i.e. there is no universal correct answer.
My preference in these matters is to stick exactly to the position title, so I would say "Dear Asst. Prof. Smith", or "Dear Assoc. Prof. Smith" in a first email. Thereafter you can use more informal salutations if appropriate.
It is true that in some countries (e.g., USA) you can just call them all "professor", but it is somewhat odd to do this in certain countries. For example, in Australia we generally reserve the term "professor" only for a full professor. If I write an email to an Associate Professor here, I always call them "Assoc Prof." not "Prof." In fact, I sometimes get emails fram academics in the US addressing me as "Professor O'Neill" and I don't like it because it feels like I am being given a title I have not earned. For that reason, I think it is best to stick exactly to the exact position title, neither inflating nor deflating the title.