Emil Hans Willi Hennig (20 April 1913 – 5 November 1976) was a German biologist and zoologist who is considered the founder of phylogenetic systematics, otherwise known as cladistics.

This was based on the conviction that 'phylogenetic systematics would lose all ground on which it stands' if the presence of apomorphous characters in different species were considered first of all as convergences (or parallelisms), with proof to the contrary required in each case.

[7] He is also remembered for Hennig's progression rule in cladistics, which argues controversially[8] that the most primitive species are found in the earliest, central part of a group's area.

Willi had two brothers, Fritz Rudolf Hennig, who became a minister, and Karl Herbert, who went missing at the Battle of Stalingrad in 1943.

Hennig saw him regularly until van Emden was expelled from National Socialist Germany for having a Jewish mother and wife.

Hennig concluded his studies with a dissertation entitled, Beiträge zur Kenntnis des Kopulationsapparates der cyclorrhaphen Dipteren.

On 1 January 1937, he obtained a scholarship from the Deutsche Forschungsgemeinschaft (DFG) to work at the German Entomological Institute of the Kaiser-Wilhelm-Gesellschaft in Berlin-Dahlem.

He was injured by grenade shrapnel in 1942 and was subsequently used as entomologist at the Institute for Tropical Medicine and Hygiene in Berlin, carrying the rank of a Sonderführer (Z).

Through his active participation in war as soldier and scientist Hennig was later subjected to accusations that he had been a member of the National Socialist party, especially by the Italian biologist and founder of panbiogeography, Leon Croizat.

Moving to East Berlin was out of the question, as Hennig held anti-communist views and already had a troubled relationship with the Sozialistische Einheitspartei Deutschlands (Socialist Unity Party of Germany, SED) that was the ruling political party of East Germany, as Hennig had repeatedly helped employees of the institute gain employment in the West.

He rejected offers from the U.S. Department of Agriculture in Washington, D.C., and an offer made by his friend Elmo Hardy, to become a Research Fellow at the University of Hawaii in Honolulu, citing as reasons that the education of his sons took priority for him, and that he needed to have the "cultural witnesses of the antique Greek-Roman Europe within ready access".

Significant are the review articles published in Erwin Lindner's Flies of the Palaearctic Regions and the Handbuch der Zoologie.

Willi Hennig only visited international institutions abroad twice, in spite of receiving many invitations for guest lectures.

His stay in Canada was also used for visits to various entomological collections in museums of the US, including Cambridge, Chicago, Washington, D.C., and New York, always in the hope of finding further amber inclusions of Dipterans, that featured prominently in his research of the late 1960s and early 1970s.

On the initiative of students whom he had lectured on several animal taxa, Hennig was made an honorary professor at the University of Tübingen on 27 February 1970.

[14] The Willi Hennig Society, an organization devoted to the advancement of cladistic principles in systematic biology, was founded in 1981.