This is a randomised, open label multi-centre phase III study comparing the activity of lapatinib alone versus trastuzumab alone versus trastuzumab followed by lapatinib versus lapatinib concomitantly with trastuzumab in the adjuvant treatment of patients with ErbB2 overexpressing and/or amplified breast cancer. Patients will be enrolled according to one of two design schemas, with Design 2 having two chemotherapy options (Design 2 and 2B), and will be randomised to one of four treatment regimens within each design schema.