Since the Middle Ages, there has been considerable evolution and proliferation in the number and types of doctorates awarded by universities throughout the world, and practices vary from one country to another. While a doctorate usually entitles one to be addressed as "doctor," usage of the title varies widely, depending on the type of doctorate earned and the doctor's occupation.
Broadly speaking, doctorates may be loosely classified into the following categories:
Research doctorates
Research doctorates are awarded in recognition of academic research that is (at least in principle) publishable in a peer-refereed academic journal. In many countries, including the United States, earning a research doctorate also requires successful completion of a regimen of coursework beyond the masters level. The best-known degree of this type, in the Anglophone world, is that of Doctor of Philosophy (Ph.D., or sometimes D.Phil) awarded in many countries throughout the world. Others include the degree of Doctor of Education, various doctorates in engineering, such as the US Doctor of Engineering (also awarded in Japan and South Korea), the UK Engineering Doctorate and the German Engineering Doctorate Doktor-Ingenieur and the German degree of Doctor rerum naturalium (Dr.rer.nat.). The Doctor of Theology, often stylized Th.D., is also a research doctorate, in theology, awarded by universities such as Harvard Divinity School and the University of Toronto among many others. Likewise, the Doctor of Sacred Theology is also a research doctorate in theology, but particular to Catholic Pontifical Universities and Faculties.
Criteria for award of research doctorates vary somewhat throughout the world, but typically requires the submission of a substantial body of original research undertaken by the candidate. This may take the form of a single thesis or dissertation, or possibly a portfolio of shorter project reports, and will usually be assessed by a small committee of examiners appointed by the university, and often an oral examination of some kind. In some countries (such as the US) there may also be a formal taught component, typically consisting of graduate-level courses in the subject in question, as well as training in research methodology.
The minimum time required to complete a research doctorate varies by country, and may be as short as three years (excluding undergraduate study), although it is not uncommon for a candidate to take up to ten years to complete.
In UK an equivalent formation to doctorate is the QCF 8.9
Higher doctorates
In some countries, especially the United Kingdom, Ireland, Australia and some Scandinavian nations, or former USSR and other Eastern Bloc countries, there is a higher tier of research doctorates, awarded on the basis of a formally submitted portfolio of published research of a very high standard. Examples include the Doctor of Sciences (DSc/ScD) and Doctor of Letters (DLitt/LittD) degrees found in the UK, Ireland and some Commonwealth countries, and the traditional doctorates in Denmark and Norway, including Dr. Theol. (Theology), Dr. Jur. (Law), Dr. Med. (Medicine) and, after Denmark and Norway introduced the Ph.D. at a lower level, Dr. Phil(os).. The Danish and Norwegian titles should not be confused with German doctorates.
The French, German and Polish habilitation (a formal professorial qualification with thesis and exam) is commonly regarded as belonging to this category. However, in some German states, the Habilitation is not an academic degree, but rather a professorial certification ("facultas docendi") that the person concerned holds all the qualifications needed to teach independently at a German university. In other German states, the "Habilitand" is awarded a formal "Dr. habil." degree. In some cases where such degree is awarded, the regarding person may add "habil." to his or her research doctorate such as "Dr. phil. habil." or "Dr. rer. nat. habil." The French academical system used to have a higher doctorate, called "State doctorate" (doctorat d'État) but it was supersed by the habilitation in 1984.
In Sweden, a title roughly corresponding to the Habilitation is Docent. This was also commonly used in Poland but as of 2005 was changed to a formal "Dr hab." (doktor habilitowany) degree. See below (Poland section) for more details.
Higher doctorates are often also awarded honoris causa when a university wishes to formally recognize an individual's achievements and contributions to a particular field.
Professional doctorates
Professional doctorates are awarded in certain fields where scholarly research is closely aligned with a particular profession, such as law, medicine, or psychology. Examples include the US and Canadian degrees of Doctor of Osteopathic Medicine (D.O.), Medicinae Doctor (M.D.), Doctor of Chiropractic (D.C.), Doctor of Dental Medicine (D.M.D), Doctor of Dental Surgery (D.D.S.), Doctor of Nursing Practice (D.N.P.), Doctor of Pharmacy (Pharm.D.), Juris Doctor (J.D.) and Doctor of Optometry (O.D.), and the Czech and Slovak degrees of Doctor of Medicine (MUDr. – Medicinae Universae Doctor) and Doctor of Veterinary Medicine (MVDr. – Medicinae Vetenariae Doctor).
Professional doctorates originated in the United States, with the introduction of the MD at Columbia University in 1767, or almost 100 years before a research doctorate (that is, a PhD) was awarded in that country, at Yale in 1861. The JD was introduced in 1870, just a few years after the PhD.
The term Professional Doctorate is used to refer to research doctorates with a focus on applied research, or research as used for professional purposes. Among others, these include the degrees of Doctor of Business Administration (DBA), Doctor of Strategic Leadership (DSL) Doctor of Education (Ed.D), Doctor of Public Administration (DPA), Doctor of Biblical Studies (D.B.S.), Doctor of Law and Policy (Lp.D), Doctor of Occupational Therapy (O.T.D.), Doctor of Practical Theology (DPT), Doctor of Professional Studies (DPS or DProf), Doctor of the Built Environment (DBEnv) and some others in various specified professional fields.
In Australia, the term is on occasion applied to the SJD, and on other occasions that degree is also categorized as a research degree.
Honorary doctorates
When a university wishes to formally recognize an individual's contributions to a particular field or philanthropic efforts, it may choose to grant a doctoral degree honoris causa (i.e., "for the sake of the honor"), the university waiving the usual formal requirements for bestowal of the degree. Some universities do not award honorary degrees, for example, Cornell University, the University of Virginia, and Massachusetts Institute of Technology.
Professor as a higher degree
An extreme rarity among degrees are the Professorial degrees.
In modern times, the status of Professor is awarded as a recognition of sustained academic excellence, equivalent in standing to an honorary doctorate, but this is not a degree per se. However, in past times, Professor was sometimes awarded as a degree.
One example of this is the degree of Sacrae Theologiae Professor (STP), which was awarded by the Pontifical University. This degree is now titled Sacrae Theologiae Doctor (STD) in keeping with usual modern practices.