Only about 60 Anopheline species transmit malaria among more than 3,000 mosquito species recorded in the world. In Africa, the major vectors are Anopheles gambiae,An. arabiensis, An. funestus, An. nili and An. moucheti. They all belong to species complexes or groups of closely related species that are very difficult to set apart on morphological grounds, but which may have highly variable behaviours and vectorial capacities. Understanding this complexity is of major importance in vector control programs or for implementing any public health intervention program such as drugs or vaccine trials. Among the seven species of the complex,Anopheles gambiaes.s. shows a huge chromosomal polymorphism related to adaptation to specific natural or anthropic environments, from equatorial forested Africa to dry sahelian areas. Recent studies conducted in West and Central Africa suggest an incipient speciation into 2 molecular forms provisionally called M and S. A similar evolutionary phenomenon is observed in An. funestus, in which sympatric populations carrying specific chromosomal paracentric inversions showed restricted gene flow. Distribution of species from An. nili group and An. moucheti complex is restricted to more humid regions of Africa. However in some areas these species play the major role in malaria transmission. Comprehensive knowledge of transmission cycles and of behavioural and underlying genetic heterogeneities that exist within and among natural vector populations will thus benefit the whole area of malaria control and epidemiology. Molecular and genetic studies, as well as in depth monitoring of vector biology, have been recently facilitated by advances in functional and comparative genomics, including recent publication of the nearly complete genome sequence of An. gambiae. Challenge for the next years is to answer to the very simple question: why is an insect a vector?