In the United States, the assistant professor is often the first position held in a tenure track, although it can also be a non-tenure track position. Full professorships are assistant professor, associate professor, and full professor in order. After 7 years, if successful, assistant professors can get tenure and also get promoted to associate professor.
It is very competitive to become a tenure-track assistant professor, especially at top-tier and research universities in the U.S., U.K., and, for example, countries like Netherlands, Germany, Switzerland, and Sweden. Often hundreds of applicants apply for a single position. Due to funding issues, the number of positions for full-time professors (either assistant or associate) has dropped significantly. Colleges are saving money by replacing full-time professors with adjuncts. With these facts, less than 20% of doctoral graduates get tenure-track assistant professor positions after graduation.
The table presents a broad overview of the traditional main systems, but there are universities that use a combination of those systems or other titles. Some universities in Commonwealth countries have also entirely adopted the North American system in place of the Commonwealth system.