Magnus Hirschfeld was the founder, director, and animating intelligence of the Institut. A physician, he had been drawn into the campaign for homosexual law reform through the Scientific Humanitarian Committee, which he co-founded in 1897 and which launched the first petition for the repeal of Paragraph 175, the German law criminalising male homosexuality. His scientific work was in the service of this advocacy: the collection of data on sexual variation, the demonstration of its natural basis, and the consequent argument that legal persecution was medically and ethically indefensible. His monumental Die Homosexualität des Mannes und des Weibes (1914) remained the most comprehensive clinical and social study of homosexuality for decades. The Institut’s motto — Per Scientiam ad Justitiam, Through Science to Justice — was his personal formulation.

Arthur Kronfeld was the Institut’s chief psychiatrist and one of the most sophisticated medical psychologists of the Weimar period. He brought a phenomenological and philosophical rigour to the study of sexual and psychological variation that complemented Hirschfeld’s more biological approach. His work on the theory of psychotherapy and on the relationship between psychiatry and philosophy placed him in the intersection of clinical medicine and theoretical psychology. He emigrated to the Soviet Union after the Nazi seizure of power, where he continued his academic career; he died in 1941, a suicide, as the German army approached Moscow.

Felix Abraham was the physician at the Institut who performed and documented the earliest gender reassignment surgeries for which systematic clinical records survive. His 1931 paper in the Zeitschrift für Sexualwissenschaft, documenting two cases of vaginoplasty, is among the foundational documents of transgender medicine. He died in 1931, before the Institut’s destruction, at the age of thirty. His clinical work was made possible by the Institut’s combination of medical facilities, research commitment, and ideological support for the recognition of transgender identity as a legitimate subject of medical care.

Max Hodann was a physician and sex educator who worked at the Institut and was among the most committed advocates of sex education as a public health and social justice measure. His popular writings on sexual health, contraception, and sexual development were addressed to working-class audiences and combined medical information with a socialist political framework that challenged the taboos surrounding sexuality as instruments of social control. He was active in the Spanish Civil War as a medical officer, worked as a physician for the Spanish Republic, and died in Stockholm in 1946.

Harry Benjamin studied under Hirschfeld in Berlin before emigrating to the United States, where he developed the first systematic clinical approach to what he called transsexualism. His book The Transsexual Phenomenon (1966) was the foundational clinical text of transgender medicine as it developed in the United States, establishing the diagnostic category, the clinical criteria for treatment, and the rationale for hormonal and surgical intervention. The Harry Benjamin International Gender Dysphoria Association, now the World Professional Association for Transgender Health, carried his name until 2007. He practised medicine in New York until his final years and died at 101.

Kurt Hiller was a writer, political activist, and legal reformer who was among the most vocal advocates for the repeal of Paragraph 175 in the Weimar period. A member of the Scientific Humanitarian Committee and associated with the Institut, his Der Aufbruch zum Paradies and his political journalism argued for the decriminalisation of homosexuality from a libertarian and rationalist rather than merely a medical basis. He was arrested by the SA in 1933, survived the concentration camps, and emigrated to London, where he continued his political activity.

Helene Stöcker was a feminist philosopher, pacifist, and sexual reformer whose work intersected with Hirschfeld’s at multiple points. She founded the Bund für Mutterschutz (League for the Protection of Mothers) in 1905, which advocated for unmarried mothers, reproductive rights, and the reform of the legal framework governing sexuality and family. Her concept of the ‘New Ethic’ argued that sexuality should be governed by love and consent rather than by legal marriage, and that the double standard of sexual morality was a form of oppression requiring both legal and philosophical reform. She was an absolute pacifist who refused military service advocacy in any form, a position that brought her into conflict with parts of the socialist movement as well as with the right.

Karl Giese was Magnus Hirschfeld’s partner and the Institut’s archivist. He worked at the Institut from the early 1920s, managing its collections and its practical administration, and accompanied Hirschfeld into exile after 1933. He died in Brno in 1936, a year after Hirschfeld’s death. His testimony, and the letters and documents he preserved, constitute important evidence for the Institut’s internal life and for the experience of its destruction.

Dora Richter was among the first people to undergo complete gender reassignment surgery in the modern period, with operations performed at the Institut in 1930 and 1931. She lived at the Institut as a domestic employee and was present when the SA raid took place in May 1933. Her fate after the raid is unknown. Her name appears in Hirschfeld’s published cases and in the Institut’s records; she is remembered as one of the clearest human embodiments of the work the Institut did and of what was destroyed when it was burned.