Non-small cell carcinomas are divided into three main categories according to the predominant morphology of the tumour cells as determined by light microscopy. Although it is possible to distinguish different histological sub-groups in this way, or by using gene expression profiling, treatment is currently decided on the basis of tumour staging, age and performance status and is independent of tumour histology. In the future, this may change as new drugs are developed and as more information is gathered on the predictive power of such expression profiles.