The year 1866 in science and technology included many events, some of which are listed here.

Astronomy

• Giovanni Schiaparelli realizes that meteor streams occur when the Earth passes through the orbit of a comet that has left debris along its path.
• William Huggins studied the spectrum of a nova and discovers that it is surrounded by a cloud of hydrogen.

Biology

• Gregor Mendel published his laws of inheritance.
• Ernst Haeckel challenged the plant/animal division of life, observing that single celled organisms, the protists, do not fit into either category.
• Frederick Smith first discovers Formica candida in the Bournemouth, England, describing it as Formica gagates.

Chemistry

• Dynamite invented by Alfred Nobel.
• Augustus von Hofmann proposes the now standard system of hydrocarbon nomenclature.

Medicine

• Max Schultze discovers two sorts of 'receptors' in the retina.
• Patrick Manson starts a school of tropical medicine in Hong Kong

Awards

• Copley Medal: Julius Plücker
• Wollaston Medal for Geology: Charles Lyell

Births

• September 25 - Thomas Hunt Morgan (d. 1945), biologist, Nobellaureate in Physiology or Medicine
• November 30 - Robert Broom (d. 1951), paleontologist

Deaths

• June 20 - Bernhard Riemann (b. 1826), mathematician.
• December 1 - George Everest (b. 1790), surveyor and geographer.