George Simms Hammond (May 22, 1921 – October 5, 2005) was an American scientist and theoretical chemist who developed "Hammond's postulate", and fathered organic photochemistry,–the general theory of the geometric structure of the transition state in an organic chemical reaction. Hammond's research is also known for its influence on the philosophy of science. His research garnered him the Norris Award in 1968, the Priestley Medal in 1976, the National Medal of Science in 1994, and the Othmer Gold Medal in 2003. He served as the executive chairman of the Allied Chemical Corporation from 1979 to 1989.

He was a chemist at the California Institute of Technology, and subsequently headed both the Departments of Chemistry and Chemical Engineering at the university. He conducted research at the University of Oxford and University of Basel as a Guggenheim Fellow and National Science Foundation Fellow, respectively. He served as the foreign secretary of the National Academy of Sciences from 1974 to 1978.

A native of Maine, he was born and raised in Auburn; he attended nearby Bates College in Lewiston, Maine, where he graduated magna cum laude with a B.S. in chemistry in 1943. He completed his doctorate at Harvard University in 1947, under the mentorship of Paul Doughty Bartlett, and a postdoctorate at University of California, Los Angeles with Saul Winstein in 1948.

George Simms Hammond was born on May 22, 1921, in Auburn, Maine. Growing up in Auburn his family were charged with the operation of the neighborhood dairy farm on Hardscrabble Road. His father died when Hammond was thirteen. He was the oldest of seven children and was raised by a single mother. From an early age Hammond was charged with running the day-to-day operations of the dairy farm with his mother and older siblings. Hammond's parents were college graduates, but disliked the local schools in Auburn. As a result, he was homeschooled until the sixth grade. Afterwards, he was educated at various Auburn public schools before graduating in 1938. After graduating he took a gap year to continue operating his dairy farm. After his educational hiatus he applied to and was accepted into Bates College, in Lewiston, Maine. He graduated with a Bachelors of Science in chemistry magna cum laude and Phi Beta Kappa in January 1943.

Upon graduating from college, Hammond took a position as a chemist at Rohm and Haas in Philadelphia, Pennsylvania. After some months on the job he quit to pursue graduate studies at Harvard University, where he received a Masters of Science (M.S.) and Doctor of Philosophy (Ph.D.). His thesis, Inhibition of the Polymerization of Allylacetate, was supervised by Paul Doughty Bartlett. Hammond then moved to Los Angeles, California, to study intermolecular compounds at UCLA.

His academic career began in 1948 with a teaching position at Iowa State College; he served as Assistant Professor of Chemistry. In his capacity there he published his eponymous postulate which is now widely known as the most important publication in the field of organic photochemistry. He moved to the University of Oxford and University of Basel as a Guggenheim Fellow and National Science Foundation Fellow, respectively. In 1958, he moved to the California Institute of Technology as a Professor of Organic Chemistry. Later he was appointed the Arthur Amos Noyes Professor of Chemistry and subsequently went on to lead the Departments of Chemistry and Chemical Engineering. After 14 years teaching and serving as an academic administrator at Caltech he moved in 1972 to the University of California Santa Cruz. At University of California Santa Cruz he served as both a professor and the Chancellor of the natural sciences.

Aside from the academic world, during all these years, George Hammond, "made many public speeches on controversial themes, both political (e.g., the invasion of Cambodia, delivered in 1971 at a public rally on Caltech's Olive Walk) and scientific (e.g., the future of chemistry)" Many of these controversial speeches affected his career negatively. For example, after his speech at Olive Walk, president Richard Nixon's administration removed his name from nomination for a major NSF post. Nevertheless, he did not back down and continued to criticize the government, and not limiting to delivering speeches, he wrote a letter to the editor of a newspaper saying: “A June 30 front-page article describes the potential bonanza in arms sales to new members as the North Atlantic Treaty Organization expands. I was favorably inclined toward expansion because of my naive assumption that bringing most of the nations of Europe and North America together as a cooperating group would decrease the likelihood of war. I cannot believe this will be the case if a prerequisite for entry is that countries buy new armaments from present members. At whom will the guns be aimed? Russia? Then we will probably re-create the cold war." The way this excerpt was written says many things about George Hammond, starting with his passionate character. Hammond fought for everything he believed in. He cared about his nation and he was also a little reckless about the consequences he could suffer for defying the government. Also, in the excerpt, a sarcastic side of Hammond can be perceived, a man of strong character with the ability to recognize when he is wrong.

He was appointed as the Foreign Secretary of the National Academy of Sciences in 1974 and served for one term retiring in 1978. He also gave notable speeches on political issues such as the invasion of Cambodia, and various topics on Chemistry. The talks he gave sometimes had negative impacts on his life, exemplified by Nixon's withdrawal of his name for major National Science Foundation positions. In 1979 he retired from academia and joined the Allied Chemical Corporation as Executive Chairman, serving for ten years. He retired from this capacity and all others after his tenure concluded.