Herbert M. Parker (13 April 1910, Accrington – 5 March 1984, Richland, Washington) was an English, and American immigrant, medical physicist.
He was a pioneer of medical radiation therapy and radiation safety, known for introducing the roentgen equivalent physical (rep), the forerunner of the gray and the rad, and also the roentgen equivalent biological (reb), the forerunner of the rem and the sievert.
With James R. Paterson, he developed in 1932 the Paterson-Parker method, or Manchester System, for radiation therapy.
In 1942 Parker went to the University of Chicago to work on the Manhattan Project at the Metallurgical Laboratory.
In 1944 Parker returned to the state of Washington to initiate the health physics program at the Hanford Engineer Works.