HER gene family (HER1-HER4) encodes structurally similar transmembrane proteins (EGFR, HER2, ErB-3, and ErB-4) with tyrosine kinase activity. Dimerised on binding with a number of ligands, including epidermal growth factor (EGF) and transforming growth factor alpha (TGFalpha), these proteins stimulate epithelial cell proliferation. HER2 and EGFR overexpression is detected in the cells of many tumours, mainly in breast, lung and oral cancer and may be connected with HER2 gene amplification or point mutations as well as with the presence of overactive polymorphic forms of HER1 gene. The first medication of a proved efficacy in breast cancer treatment was trastuzumab (Herceptin)--monoclonal antibody against HER2 protein. Trastuzumab was effective only in the case of patients with high HER2 expression evaluated by immunohistochemical methods and with gene amplification ascertained by fluorescence in situ hybridisation assays. In non-small-cell lung cancer (NSCLC), HER2 overexpression was detected only in a few cases. Therefore, trastuzumab treatment seems to be problematic in NSCLC patients. A small molecule quinazoline (erlotinib, Tarceva) is a promising therapeutic agent selectively blocking EGFR. Phase III Tarceva clinical trail in NSCLC patients showed that their survival is prolonged and that the medication acts together with other chemotherapeutic agents like cisplatin and gemcitabine.