The year 1866 in science and technology involved some significant events, listed below.

• May – William Huggins studies the emission spectrum of a nova and discovers that it is surrounded by a cloud of hydrogen.^[1]
• June 4 – Pluto (not known at this time) reaches its only aphelion between 1618 and 2113.
• Giovanni Schiaparelli realizes that meteor streams occur when the Earth passes through the orbit of a comet that has left debris along its path.

• Gregor Mendel publishes his laws of inheritance.^[2]
• Ernst Haeckel challenges the plant/animal division of life, observing that single celled organisms, the protists, do not fit into either category.
• Élie Metchnikoff describes the early separation of "polecells" (progenital cells) in parthenogenetic Diptera.^[3]
• Robert John Lechmere Guppy discovers the guppy (fish) in Trinidad.
• Frederick Smith first discovers Formica candida in the Bournemouth district of England, describing it as Formica gagates.
• Nikolai Kaufman publishes his Moscow Flora.^[4]

• Dynamite invented by Alfred Nobel.^[5]
• August von Hofmann proposes the now standard system of hydrocarbon nomenclature and invents the Hofmann voltameter.^[6]
• Emil Erlenmeyer proposes that naphthalene has a structure of two fused benzene rings.^[7]

• January 26 – Volcanic eruption in the Santorini caldera begins.

• The second smallest pair of amicable numbers (1184, 1210) is discovered by teenager B. Nicolò I. Paganini.

• February 21 – Lucy Hobbs Taylor becomes the world's first woman to receive a doctorate from a dental college (Ohio College of Dental Surgery).^[8]
• July – Elizabeth Garrett Anderson opens the St Mary's Dispensary in London where women can seek medical advice from exclusively female practitioners.^[9]
• Max Schultze discovers two sorts of 'receptors' in the retina.^[10]
• Dr John Langdon Down publishes his theory that different types of mental condition can be classified by ethnic characteristics, notably "Mongolism", the genetic developmental disability now known as Down syndrome.^[11][9]
• Invention of a clinical thermometer by Thomas Clifford Allbutt.^[9]
• A cholera epidemic in London causes over 5,000 deaths.^[9]
• Patrick Manson starts a school of tropical medicine in Hong Kong.^{[citation needed]}

• American paleontologist Joseph Leidy describes the new genus and species Laelaps aquilunguis, demonstrating that theropod dinosaurs walked on their hind limbs rather than on all fours as in earlier reconstructions.^[12]

• James Clerk Maxwell formulates the Maxwell–Boltzmann distribution in the kinetic theory of gases.

• January 12 – Royal Aeronautical Society is formed as 'The Aeronautical Society of Great Britain' in London, the world's oldest such society.
• July 27 – The SS Great Eastern successfully completes laying the transatlantic telegraph cable between Valentia Island, Ireland and Heart's Content, Newfoundland, permanently restoring a communications link.^[13]
• August 23 – Ralph H. Twedell patents the hydraulic riveter in the United Kingdom.^[14]

• Copley Medal: Julius Plücker^[15]
• Wollaston Medal for Geology: Charles Lyell

• February 1 – Agda Meyerson (died 1924), Swedish nurse and healthcare profession activist
• February 8 – Moses Gomberg (died 1947), Russian-born chemist
• February 14 – Victor Despeignes (died 1937), French pioneer of radiation oncology
• February 26 – Herbert Henry Dow (died 1930), Canadian-born chemist
• April 17 – Ernest Starling (died 1927), English physiologist
• July 13 – Emily Winifred Dickson (died 1944), Irish-born gynaecologist
• July 25 – Frederick Blackman (died 1947), English plant physiologist
• September 13 – Arthur Pollen (died 1937), English inventor
• September 21 – H. G. Wells (died 1946), English scientific populariser
• September 25 – Thomas Hunt Morgan (died 1945), American biologist, Nobel laureate in Physiology
• October 8 – Reginald Fessenden (died 1932), Canadian pioneer of radio broadcasting
• November 11 – Martha Annie Whiteley (died 1956), English chemist and mathematician
• November 30 – Robert Broom (died 1951), Scottish-born paleontologist
• December 7 – Maude Delap (died 1953), Irish marine biologist

• March 6 – William Whewell (born 1794), English scientist, philosopher and historian of science
• March 14 – Alexander Morison (born 1779), Scottish physician and psychiatrist
• April 4 – William Dick (born 1793), Scottish veterinarian
• April 5 – Thomas Hodgkin (born 1798), English physician
• July 20 – Bernhard Riemann (born 1826), German-born mathematician
• September 16 – François Mêlier (born 1798), French physician ^[16]
• October 18 – Philipp Franz von Siebold (born 1796), German physician, botanist and traveler in Japan
• December 1 – George Everest (born 1790), British surveyor and geographer