Dr. Henry Newell Guernsey was a homeopathic physician, surgeon, and obstetrician who practiced in Philadelphia from 1844 until April 1885. He graduated from the New York University College of Medicine in 1844 and moved to Philadelphia where he began practicing medicine. In his career, Dr. Guernsey was a member of many influential homeopathic societies and organizations, and served as a professor at the Homeopathic Medical College of Pennsylvania in both Obstetrics and Materia Medica. He was also the Dean of Faculty at the college from 1866-1869 and 1871-1874. Dr. Guernsey was one of the key innovators of the “keynote system” of homeopathic prescription. He died from pulmonary tuberculosis on June 27, 1885. The Henry Newell Guernsey collection consists mainly of correspondences and reference materials, mostly dating from the 1860s-1884, with some material from the 1850s. The correspondences mainly pertain to the practice and state of homeopathy, the projects of Dr. Guernsey and others, and the administrative workings of the Homeopathic Medical College of Pennsylvania. There are also considerable correspondences written by and addressed to his son, Dr. Joseph Collourn Guernsey. The reference materials mainly pertain to the history and spread of homeopathy in North America, but there are also a large number of materials regarding the organization and operation of many homeopathic societies and organizations Dr. Guernsey was a member of. This collection would be useful to researchers studying the history and practice of homeopathy in the mid 19th century, due to the amount of material on the subject compiled by Dr. Guernsey. This collection would also be of use to researchers studying the major homeopathic societies and organizations of the time, as there are many materials documenting meetings, administrative aspects, and organization of these groups.