HER-2/neu (human epidermal growth factor receptor) is involved in cell survival, proliferation, angiogenesis and invasiveness and is overexpressed in about 25% of breast cancers. Overexpression of HER-2 is associated with response to the anti-HER-2 antibody Trastuzumab (Herceptin). However HER-2 expression can be heterogeneous within the primary tumour and exhibit discordance in expression between a primary tumour and its metastasis bringing into question the practise of HER-2 screening for trastuzumab candidate patients using material obtained only from the primary tumour. Medical imaging modalities using HER-2 targeted tracers (or contrast agents) facilitate a global approach to the determination of HER-2 expression across all detectable tumour lesions carried by the patient and could provide a more reliable indication of the patient’s likely response to trastuzumab treatment. Here is reviewed the development and preclinical (and occasional clinical) assessment of HER-2 targeted tracers along with studies detailing the use of established imaging tracers such as 11C-choline in the determination of response to tratsuzumab for a range of medical imaging modalities including PET (positron emission tomography), SPECT (single photon emission tomography), MRI and optical imaging.