Viral transformation is the change in growth, phenotype, or indefinite reproduction of cells caused by the introduction of inheritable material.  Through this process, a virus causes harmful transformations of an in vivo cell or cell culture.  The term can also be understood as DNA transfection using a viral vector. 
Viral transformation can occur both naturally and medically.  Natural transformations can include viral cancers, such as human papillomavirus (HPV) and T-cell Leukemia virus type I.  Hepatitis B and C are also the result of natural viral transformation of the host cells.  Viral transformation can also be induced for use in medical treatments.
Cells that have been virally transformed can be differentiated from untransformed cells through a variety of growth, surface, and intracellular observations.  The growth of transformed cells can be impacted by a loss of growth limitation caused by cell contact, less oriented growth, and high saturation density. Transformed cells can lose their tight junctions, increase their rate of nutrient transfer, and increase their protease secretion.  Transformation can also affect the cytoskeleton and change in the quantity of signal molecules.