The importance of targeted therapies has been emphasized by clinical trials using antiangiogenic or HER2 inhibitors in breast cancer. First with trastuzumab, it was demonstrated that targeted therapies may improve outcome in patients with HER overexpressing breast cancer in metastatic or adjuvant settings. The emerging role for angiogenesis inhibitors has also been demonstrated with bevacizumab. Unfortunately, there is growing clinical and biological evidence that tumour cells may develop unexpected and complex mechanisms of resistance to those targeted therapies. This review outlines the mechanisms by which tumour cells may resist to new targeted agents. Most recent developments are also highlighted.