The mechanisms of stereoselectivity and chemoselectivity in the Ziegler-Natta polymerizations of conjugated dienes have been investigated by density functional methods, in the framework of the widely accepted polymerization scheme involving anti-allyl coordination of the growing chain. Most insertions would occur through intermediates with the diene monomer s-cis eta (4) coordinated to the metal and with the monomer concavity oriented toward the terminal allyl g-roup of the growing chain. Monomer coordinated intermediates presenting a backbiting of the anti-allyl coordinated growing chain (eta (3)eta (2)) would essentially lead to 1,4-unlike insertions only, while monomer coordinated intermediates simply presenting an anti-allyl coordination of the growing chain would lead to 1,4-like and/or 1,2-unlike insertions. These models are able to rationalize several experimental data relative to stereos electivity and chemoselectivity in Ziegler-Natta polymerizations of conjugated dienes.