 This is an open-label, randomised phase III, multicentre, Italian study designed to investigate the role of trastuzumab in patients with locally advanced andor metastatic breast cancer over expressing HER2 who received a first line chemotherapy containing trastuzumab. The trial in divided in two studies: 1. The maintenance study will be conducted in patients not progressing to a first line chemotherapy plus trastuzumab. 2. The 2nd line study will be conducted in patients progressing to a first line chemotherapy plus trastuzumab. 