During the course of therapy, patients with small cell lung cancer have been noted to develop transformation to non-small cell lung cancer and conversely, patients with non-small cell lung cancer have had transformation to small cell lung cancer or other non-small cell histologies. Transformation may occur after prior tyrosine kinase inhibitors, chemotherapy, immunotherapy or radiation therapy. These changes reflect on the overlapping biology of these cell types and the clinical need for re-biopsy at times of disease progression. The optimum therapy after transformation will depend upon prior therapies received, the functional capacity of the patient, and further research to define the best therapy options.