Most clinical research that involves the testing of a new intervention progresses in an orderly series of steps, called phases. Clinical trials are usually classified into one of three phases:

• Phase I trials: These are the first studies to look at how a new intervention works in people -- the manner and frequency of its application and, if it's a drug, what dose is safe. A phase I trial usually enrolls only a small number of participants, sometimes as few as a dozen.
  
  
• Phase II trials: A phase II trial continues to test the safety of the intervention, and begins to evaluate how well it works. Phase II studies usually focus on a particular type of cancer.
  
  
• Phase III trials: These studies test a new intervention in comparison to the current standard of care. Phase III trials often enroll large numbers of people and may be conducted at many doctors' offices, clinics, and cancer centers nationwide.

Because they build on reliable findings from earlier clinical trials, phase III trials are usually considered to be the ultimate test of a new intervention.

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