Professor in 387: Plato founds the Academy

At research universities, almost always: the doctorate is the profession's baseline credential in nearly every country. The main exceptions are professors of practice — distinguished judges, architects, artists or executives appointed to teach their craft — and some conservatoire and fine-arts traditions where a professional reputation substitutes for a doctorate. German-speaking systems historically demanded even more: a second, post-doctoral thesis called the Habilitation.

How long does it take to become a professor? Typically ten to fifteen years after secondary school: a bachelor's degree of three to four years, a doctorate of four to seven, then two to six years of postdoctoral or fixed-term posts before a first permanent appointment. Tenure or an equivalent permanent status usually arrives in a candidate's mid-to-late thirties, and in German-speaking and some Asian systems often later still.

What is tenure, and why does it matter? Tenure is an indefinite appointment that can be revoked only for serious cause, designed to protect scholars who publish unpopular findings. The American standard was set by the AAUP's 1940 Statement of Principles, typically after a six-year probation and an up-or-out review. Other systems protect professors differently: German professors are civil servants, while the UK abolished formal tenure in its 1988 Education Reform Act.