Lapatinib is an active and well-tolerated oral dual tyrosine kinase inhibitor that is able to inhibit epidermal growth factor receptor (EGFR) and HER2 with high specificity. It has shown promising activity in several cancers most notably metastatic breast cancer. Lapatinib is active in both trastuzumab-refractory and -naïve breast cancer and a number of phase III studies are ongoing to fully define its activity. Studies have also been conducted in a range of other cancers including renal cell carcinoma. It is essential that future studies incorporate predictive biomarker subgroup analysis. This will help to ensure that important subgroup activity is not missed and also allow the definition of groups most likely to benefit from lapatinib.