The majority of studies on leeches are related to: internal and external morphology. Fewer molecular research projects focus on molecular studies and karyology. The latter are needed to explain the evolutionary and systematic issues. Karyotypes for 22 species of Hirudinida have hitherto been determined, including: 8 species belonging to the Glossiphoniidae family, 3 species of Piscicolidae, 7 of Erpobdellidae, 1 of Haemopidae and 3 of Hirudinidae. The chromosome number vary among individual groups of leeches. Within Glossiphoniidae the chromosome diploid number ranges from 14 to 32, in Piscicolidae from 20 to 32, in Erpobdelliformes from 16 to 22, and in Hirudiniformes from 24 to 28. The karyological analyses were used to show phylogenetic relations between main groups of Hirudinida, and the diploid number of 16 was suggested to be a primitive value. This number tends to increase as the evolution progresses. The phylogeny scheme of leeches proposed by Mann shows the Glossiphoniidae as primitive to the Piscicolidae, and Hirudinidae as giving rise to the Haemopidae and Erpobdellidae. Those hypotheses are herewith confronted with morphological, molecular, karyological, ecological and behavioral data.