Earning a Ph.D. is supposed to demonstrate that a person has extremely advanced knowledge of a subject. Ph.D candidates have to spend a tremendous amount of time researching. They not only have to learn virtually everything there is to know on a given subject, but they must personally contribute a distinctive idea or theory to that field. Because these guidelines are so subjective, each academic institution and department within that institution comes up with its own requirements for obtaining a Ph.D. Whatever those requirements may be, however, the resounding theme will be the same: learn all you can about this subject and then tell us something we don't already know.
More than 280 American universities offer Ph.D. degrees. Although they all clearly have their own admissions process and standards of excellence, most will require high grades in the admission candidate's given field. Most universities will consider the grades earned during the pursuit of a Bachelor's degree and Master's degree. These degrees are almost always required before a candidate can advance into a Ph.D. program. The only exception is for those students who choose to attend a joint program where the Master's degree can be earned during the Ph.D. process. Other elements that are scrutinized during the admissions process are letters of recommendations, standardized test scores such as the GRE, LSAT or MCAT, prior research and work experience and the candidate's level of writing.
During a Ph.D. program a candidate will not only have to attend courses in his given field, but will also have to write and submit a thesis or dissertation on a particular subject within his given major. This subject usually has to be approved beforehand by an academic board. The candidate must then create an original piece or work or theory based on his subject, which will be guided by a committee of up to three academic mentors, and oftentimes must be defended before another academic board before it can be accepted. Because this process can be long and arduous, most Ph.D. candidates spend 2 to 3 years working on this thesis, during which time they may teach other undergraduate or graduate-level classes, but will not usually be required to enroll in any other courses themselves. Any required courses are typically completed before the thesis-writing process begins. Additional specialty courses, however, may be undertaken during this time.
Depending on the program and the individual, a Ph.D. program can take anywhere from 4 to 8 years to complete. In addition to this, a person would have spent approximately 4 years obtaining her Bachelor's degree and another 1 to 2 years obtaining her Master's degree. Naturally, Ph.D candidates who already have their Master's degrees will finish sooner than candidates who must obtain it while completing their Ph.D coursework. Some people also take a year or two off to obtain practical experience in their given fields by interning or researching for a specific company that can give them real-world experience.
Though it's not required, most universities will want Ph.D. candidates who obtained degrees in the same or a similar field as their Bachelor's and Master's degrees. For example, graduating with honors from an English undergraduate and graduate program does not prove to a school that that person would be a great fit for its biology program.