Jeannel's most important work was on the insect fauna of caves in the Pyrenees, France and in the Carpathians, Romania.

Jeannel specialised in Leiodidae (then Silphidae or Catopidae) but authored a large number of papers and works on other Coleoptera.

During the rest of his career, he added another 500 scientific publications to this total, together amounting to more than 20,000 published pages—mostly on cave insects and all illustrated by himself.

[2] Jeannel's most important contribution to taxonomy was that he realised the potential of using the genitalia for species identification and classification.

In his book l'Édéage (1955), he summarized all the insights the study of the aedeagus (the term used for the male beetle genital apparatus) had afforded him.