Phase I trials: These first studies in people evaluate how a new drug should be given (by mouth, injected into the blood, or injected into the muscle), how often, and what dose is safe. A phase I trial usually enrolls only a small number of patients, sometimes as few as a dozen.

Phase II trials: A phase II trial continues to test the safety of the drug, and begins to evaluate how well the new drug works. Phase II studies usually focus on a particular type of cancer.

Phase III trials: These studies test a new drug, a new combination of drugs, or a new surgical procedure in comparison to the current standard. A participant will usually be assigned to the standard group or the new group at random (called randomization). Phase III trials often enroll large numbers of people and may be conducted at many doctors' offices, clinics, and cancer centers nationwide

Phase IV trials:In addition, after a treatment has been approved and is being marketed, the drug's maker may study it further in a phase IV trial. The purpose of phase IV trials is to evaluate the side effects, risks, and benefits of a drug over a longer period of time and in a larger number of people than in phase III clinical trials. Thousands of people are involved in a phase IV trial.

Clinical trials are sponsored by organizations or individuals who are seeking better treatments for cancer or better ways to prevent or detect cancer.

Individual physicians at cancer centers and other medical institutions can sponsor clinical trials themselves.

The National Cancer Institute (NCI) sponsors a large number of clinical trials. The NCI has a number of programs designed to make clinical trials widely available in the United States.

Drug companies or companies that make diagnostic equipment (like X-ray machines) sponsor trials of their products, hoping to demonstrate that their products are safe and effective. The U.S. Food and Drug Administration (FDA) will only permit companies to sell a product after it has been proven safe and effective in clinical trials.