From Aristote till the 1950s, only two kingdoms were considered by taxonomists : the plant and the animal kingdoms. Yeasts were described as “unicellular plants of microscopic size” (in : Industrial Microbiology, Prescottt & Dunn, 1949).
 
In the 1960s, Yeasts were classified within the Ascomycetes, which are part of Fungi, which themselves are Thallophytes (together with algae for instance), belonging to the Vegetal Kingdom.
 
Although some discussions still exist within the scientific community,  according to the latest taxonomic studies, living organisms can be classified in 7 Kingdoms instead of 2 : 
-       Animals
-       Vegetals
-       Bacteria
-       Protista (e.g. Algae)
-       Chromista (e.g. Kelp)
-       Fungi (e.g. Yeasts)
-       Protozoa (e.g. Amoebae)
 
The Fungi Kingdom is divided in 4 phyla (groups) :
-       Ascomycota
-       Basidiomycota
-       Zygomycota
-       Deuteromycota.
 
Yeasts belong to the phylum Ascomycota, also called Sac Fungi because they produce spores in little sacs called asci. The Sac Fungi include the prestigious Morels and Truffles.