A PhD, or Doctor of Philosophy degree, is available in a wide range of specialties in the humanities, sciences, social sciences and applied sciences. Common specialties include history, biological sciences and computer science.
In the United States and Europe, the three major levels of collegiate and post-collegiate education are the Bachelor's Degree, the Master's Degree and the PhD. The levels arose out of the Medieval university, and generally correspond to different qualifications for knowledge, practice and teaching within the Catholic Church. A bachelor's degree traditionally connoted a minimum level of general education, while a master's degree qualified a recipient to practice professionally, specifically in the Church. A PhD, which remains the highest level of educational achievement in the West, was originally intended to qualify the holder to teach in a university, though today many people with PhDs hold posts outside of academia.
As universities have grown, so have the fields in which PhDs are awarded. In addition to more traditional subjects such as philosophy, mathematics and art history, PhD programs now exist in a variety of areas including engineering, sociology and new or interdisciplinary fields such as American Studies, Women's Studies and African American Studies. In the United States, a PhD takes a significant investment of time and effort, requiring five or more years of study and the production of original research in the form of a dissertation.