Klug, Aaron (1926-) is a Lithuanian-born British scientist and molecular biologist who made important advances in electron microscopy, the technique of using electrons rather than light waves to produce magnified images. Klug used electron microscopy to study viruses and molecules that play roles in living cells. He was awarded the 1982 Nobel Prize in chemistry for this work.

Klug was born in Lithuania but at the age of 2 emigrated with his parents to Durban, South Africa. He first became interested in microbiology in high school, after reading Microbe Hunters by Paul de Kruif. He entered Johannesburg's University of Witwatersrand in 1942.

Klug earned a B.S. degree in 1945. He received a scholarship to do graduate work in physics at the University of Cape Town. There he learned X-ray crystallography, a technique for determining the molecular structure of crystals. After completing his M.S. degree, he remained at Cape Town to do further research in X-ray diffraction.

Klug became increasingly interested in the structure of matter and particularly in using X-ray crystallography in what were, at the time, less traditional applications, such as to determine protein structure. In 1949, he joined Cambridge University's acclaimed Cavendish Laboratory to do his doctoral work, again with the aid of a scholarship. He had hoped to do biological research there under such eminent physicists as John Cowdery Kendrew and Max Ferdinand Perutz, who were doing work in crystallography, but there were no openings for lab assistants in their groups. Instead, he began research on the molecular structure of steel, gaining invaluable experience in solid-state physics, computing, and in understanding phase transition, all of which served him well later, in his research in the field of molecular biology.

In 1954, after earning his Ph.D. degree, Klug became a Nuffield research fellow at Birkbeck College in London and worked on the X-ray analysis of biological molecules. At Birkbeck, he worked with British physicist Rosalind Elsie Franklin, who was using X-ray crystallography to study the virus that causes the plant disease tobacco mosaic. Klug joined her team in researching the tobacco mosaic virus and over a four-year period worked out the basic structure of the tobacco-attacking virus.

After Franklin's death in 1958, Klug took over the position of director of the Virus Structure Research Group. In 1962, Klug moved to the Medical Research Council's newly established Laboratory of Molecular Biology at Cambridge. Klug helped to develop the lab to one world-renowned for its excellence. In 1986, he was appointed director of the entire laboratory.

He retired from the laboratory in 1996 and became a fellow of Peterhouse College, University of Cambridge, and conducts research on the structure of DNA binding proteins that regulate gene expression. He was the president of the Royal Society from 1995–2000.

Klug's most outstanding contribution was developing a technique called crystallographic electron microscopy, which used electrons rather than light waves to produce magnified images. X-ray crystallography alone was not adequate for determining the biological structure of all molecules. Another technique was to use the electron microscope, which was capable of huge magnifications. However, the resultant images were two-dimensional, thus not giving a truly accurate representation of a molecule's structure.

Klug's objective was to somehow combine these two procedures to arrive at an accurate three-dimensional representation. He reasoned that it might be possible to subject the micrographs, the photographic plates upon which the electron microscope imprinted its two-dimensional pictures of a substance, to X-ray crystallography themselves. By capturing views of a given substance from many different angles and then applying X-ray diffraction to each of the micrographs, the precise structure might be compiled.

Achieving success in this objective was a long and complex process, involving the most sophisticated mathematical analysis, but ultimately Klug was able to perfect the technique now known as crystallographic electron microscopy. Through this procedure the structures and functioning of numerous proteins, viruses, chromosomes, and other biological materials became accessible, a breakthrough that led not only to significant new discoveries in the field, but to whole new areas of scientific investigation as well. Klug's 1982 Nobel Prize acknowledged this achievement and also his discovery of the structures of many biologically important nuclei acid-protein complexes. He was knighted in 1988.