Doctor of Philosophy, abbreviated Ph.D. or PhD for the Latin Philosophiæ Doctor, meaning "teacher of philosophy", (or, more rarely, D.Phil., for the equivalent Doctor Philosophiæ) is an advanced academic degree. In the English-speaking world it has become the most common denomination for a research doctorate and applies to graduates in a wide array of disciplines in the sciences and humanities. The Ph.D. has become a requirement for a career as a university professor or researcher in many fields. In addition to university setting research/professor positions, many Ph.D. graduates go on to careers in government departments, NGOs, or in the private sector.

The detailed requirements for award of a Ph.D. vary throughout the world; however, there are a number of common factors. A candidate must submit a thesis or dissertation consisting of a suitable body of original academic research, which is in principle worthy of publication in a peer-refereed context. In many countries a candidate must defend this work before a panel of expert examiners appointed by the university; in other countries, the dissertation is examined by a panel of expert examiners who stipulate whether the dissertation is in principle passable and the issues that need to be addressed before the dissertation can be passed. There is usually a prescribed minimum period of study (typically two to three years full time) which must take place before submission of the thesis (this requirement is usually waived for academic staff submitting a portfolio of peer-reviewed published work).

The candidate may also be required to successfully complete a certain number of additional advanced courses relevant to their area of specialization. In some countries (the US and Canada, for example), most of the universities require coursework for Ph.D. degrees. In many other countries (especially those, such as the UK, which have a greater degree of specialization at the undergraduate level) there is no such condition in general. It is not uncommon, however, for individual universities or departments to specify analogous requirements for students not already in possession of a master's degree. Universities in the non-English-speaking world have begun adopting similar standards to those of the Anglophone Ph.D. for their research doctorates (see, for example, Bologna Process).