A Doctor of Philosophy (PhD, Ph.D., or DPhil; Latin philosophiae doctor or doctor philosophiae) is the most common degree at the highest academic level awarded following a course of study. PhDs are awarded for programs across the whole breadth of academic fields. Because it is an earned research degree, those studying for a PhD are required to produce original research that expands the boundaries of knowledge, normally in the form of a thesis or dissertation, and defend their work against experts in the field. The completion of a PhD is often a requirement for employment as a university professor, researcher, or scientist in many fields. Individuals who have earned a Doctor of Philosophy degree may, in many jurisdictions, use the title Doctor (often abbreviated "Dr" or "Dr.") with their name, although the proper etiquette associated with this usage may also be subject to the professional ethics of their own scholarly field, culture, or society. Those who teach at universities or work in academic, educational, or research fields are usually addressed by this title "professionally and socially in a salutation or conversation." Alternatively, holders may use post-nominal letters such as "Ph.D.", "PhD", or "DPhil" (depending on the awarding institution). It is, however, considered incorrect to use both the title and post-nominals at the same time.

The specific requirements to earn a PhD degree vary considerably according to the country, institution, and time period, from entry-level research degrees to higher doctorates. During the studies that lead to the degree, the student is called a doctoral student or PhD student; a student who has completed all their coursework and comprehensive examinations and is working on their thesis/dissertation is sometimes known as a doctoral candidate or PhD candidate (see: all but dissertation). A student attaining this level may be granted a Candidate of Philosophy degree at some institutions or may be granted a master's degree en route to the doctoral degree. Sometimes this status is also colloquially known as "PhD ABD," meaning "All But Dissertation."

A PhD candidate must submit a project, thesis, or dissertation often consisting of a body of original academic research, which is in principle worthy of publication in a peer-reviewed journal. In many countries, a candidate must defend this work before a panel of expert examiners appointed by the university. Universities sometimes award other types of doctorate besides the PhD, such as the Doctor of Musical Arts (D.M.A.) for music performers, Doctor of Juridical Science (S.J.D.) for legal scholars and the Doctor of Education (Ed.D.) for studies in education. In 2005 the European Universities Association defined the "Salzburg Principles," 10 basic principles for third-cycle degrees (doctorates) within the Bologna Process. These were followed in 2016 by the "Florence Principles," seven basic principles for doctorates in the arts laid out by the European League of Institutes of the Arts, which have been endorsed by the European Association of Conservatoires, the International Association of Film and Television Schools, the International Association of Universities and Colleges of Art, Design and Media, and the Society for Artistic Research.

In the context of the Doctor of Philosophy and other similarly titled degrees, the term "philosophy" does not refer to the field or academic discipline of philosophy, but is used in a broader sense in accordance with its original Greek meaning, which is "love of wisdom." In most of Europe, all fields (history, philosophy, social sciences, mathematics, and natural philosophy/sciences) other than theology, law, and medicine (the so-called professional, vocational, or technical curriculum) were traditionally known as philosophy, and in Germany and elsewhere in Europe the basic faculty of liberal arts was known as the "faculty of philosophy."