Contemporary views on molecular biology of bovine viral diarrhea virus are reviewed. Diversity of viral genome in the region coding for p125/NS23 and functions of viral proteins are described. Special emphasis is put on viral cytopathogenicity correlated with the rearrangement of nucleotide sequences of cytopathic strains of BVDV in a region coding for p125/NS23 protein. Pathogenesis of Mucosal Disease with respect to viral genome rearrangements is also reviewed.