What is the difference between an Instructor, Assistant Professor, Associate Professor and a Full Professor?

From what I've seen: about 7 years, 1-2 kids, gray hairs, some ulcers, and a loss of a sense of joy in research. (Kidding! Mostly...) Oh, and, in the US at least, the title of "associate professor" is often given only after tenure is granted whereas "assistant professor" is usually reserved for pre-tenure, but tenure-track, faculty.

The answer depends a lot on what country and what institution you're referring to.  So there's no single answer. In the US, an instructor (also sometimes called a lecturer) is someone, often without a PhD, whose only job is to teach (part-time or full-time).  Assistant, Associate, and Full professors are all fulltime positions, usually with requirements for both research and teaching (sometimes with requirements for extension or outreach). In most universities, Assistant professors are pre-tenure, and Associate and Full are post-tenure (but there are some universities that have untenured Associate professors). Wikipedia has a list of the titles of academic ranks in different countries: http://en.wikipedia.org/wiki/Academic_ranks Wikipedia has an entire article for academic ranks in the United States: http://en.wikipedia.org/wiki/Academic_rank_in_the_United_States

Here's a handy chart:

From a professor's point of view, there are very few real differences.  Most associated profs are tenured, so there's no difference in terms of external research expectations.  If you're going to slow down after tenure, then it won't make much of a difference if you're associate or full. There are several real differences, however: 1. most departments only accept letters of support for tenure cases from full professors. So you rarely get requests to write tenure letters for young professors until you hit full professor status. 2. universities "expect" you to do more service work at the department and university level. This means committees, councils, taskforces etc.  Many of these are giant timesinks, and universities tend to avoid pushing those onto young faculty for fear of negatively impacting their research productivity. So in short, what do you get when you hit full professorship? 1. a raise (probably) 2. requests for tenure letters (these take a LOT of time to write) 3. "requests" and appointments to various service committees in the university 4. various other activities depending on your standing and visibility in the research community (invites to keynotes, editors for journals, chairing conferences etc).

For most faculty positions there are three major career reviews: time of appointment, promotion with , and promotion to Full Professor. The first one, for a fresh Ph.D., is mainly based on promise, as indicated by his or her research work to date. External letters of review are usually from the candidate's committee, so they know the person well from the context of graduate school or recent work experience. With tenure comes the prospect of employment for life, so while this is also accompanied by an evaluation of future promise, it must also be backed up by solid (even extensive) evidence of research achievement, productivity, and impact. External letters from leaders in the research community are extremely important, as the candidate must have established him or herself as someone highly likely to lead the field if not now then in the future. Competence in teaching and service are also important. Usually this accompanied by a change in title from Assistant to Associate Professor, but not always, as some institutions have Assoc Profs without tenure (e.g., ). Full Professor is typically the last career review, in which the faculty member is assessed for extensive career achievement and promise for continued productivity. Evidence in leadership, particularly in research, and potential for leadership in service at the University as well as the research community is extremely important. I should mention that at the there are actually two more career reviews: sort of between the lower level of Full Professor and the upper level of Full Professor, and again to reach the exalted heights of so-called "Professor Above Scale" reserved for the most senior and distinguished members of the faculty. A Full Professor should be an internationally recognized leader in research and service, teacher, and mentor to students, and usually such individuals do take on a more significant role in the shared governance of the university – students don't see this, but senior Full Professors do all sorts of work intensive things related to the University – e.g., I served for three and a half years on a campus committee that met for 6 hours a week to review and recommend all advancements and promotions for the Berkeley faculty, and spent at least twice that time preparing dossiers for that committee meeting. It was a great deal of work largely invisible beyond the campus, but very important work to insure the continued excellence of the Faculty, from which the reputation for all universities ultimately comes.

The main difference is that most associate professors have tenure and most assistant professors don't. While promotion from assistant to associate and awarding of tenure are technically two different decisions, at most universities they are considered at the same time.

Barring the award of an endowed chair, fulls have climbed the academic ladder as high as it goes. (Note: I do not consider a move to administration a promotion. I consider it karmic retribution for something really bad you did at some point in the past.) Associates have promotion to full as a career goal/incentive. Typically, only fulls get to vote on promotions for associates. So fulls have some leverage over associates. Associates rarely have leverage over fulls. The next point has rather high variance: fulls who want to avoid committee assignments tend to have better luck dumping them on associates than the other way around. In my experience, most faculty are not particularly happy to be assigned to committee. There are a few that really relish committee work. (They're a big part of the reason that others try to avoid it at all costs.) Committees are nature's way of correcting for excessive productivity. Some fulls are more productive than some associates, and some associates are more productive than some fulls. I've seen specimens of both species that were "retired in place". I don't think there's a meaningful difference in productivity (although fulls who successfully duck committee assignments have that much more time to spend on research).

I'm sure there are differences in different institutions, departments, and so on ... but my association with this, as an adjunct to the University of Oregon Lundquist School of Business, is that adjuncts are part time, have real-world business experience, and are business people, not career academics. We aren't tenure track. We don't have PhDs. We  do tend to be older, we have built businesses, and we're teaching because we like to, not for the (sarcasm) money (/sarcasm) or the career.  With most of us there is a sense of giving back, of enjoying the sharing, and liking the stimulation of teaching young people who care.

Associate. Assistant profs is really entry-level professorship. Usually an associate has tenure, though not necessarily so.