In research, a person who is involved in running a clinical trial or research study. The investigator may help prepare and carry out the protocol (plan) for the study, monitor the safety of the study, collect and analyze the data, and report the results of the study. Investigators can include doctors, scientists, nurses, and other research staff. Some clinical trials or research studies may have more than one investigator. Usually one of the investigators is the principal investigator, who is in charge of the whole study.