Carcinogenesis is the result of a series of genetic mutations resulting in unregulated growth of a clone of cells and the development of malignant lesion that is largely monoclonal though, with the evolution of further genetic changes, there develops a degree of heterogeneity in the tumour. DNA technology, especially allelic imbalance (loss of heterozygosity) studies have identified chromosomal changes in oral carcinoma and head and neck squamous cell carcinoma (SCCHN), suggestive of the involvement of tumour suppressor genes (TSGs), particularly in chromosomes 3, 9, 11 and 17. The regions most commonly identified have included 3p, especially 3p14.2 (FHIT); 3p24, and 3p21.3, where the TSGs involved are as yet unidentified; 9p21 where p16 (INK4A/MTS-1] is the main target TSG; and 17p13 where p53 is the major target TSG. Over-expression of oncogenes, genes mainly involved in cell signalling, especially those on chromosome 11 (PRAD-1 in particular) and 17 (H-ras) and mutations in DNA repair genes, have also been implicated in the carcinogenesis of SCCHN.