Although much research has focused on using near-infrared silica-based gold nanoshells for dual-imaging and therapeutic applications in vivo, these particles may also prove useful as rapid diagnostic imaging agents for ex vivo applications, such as intraoperative tumor margin detection. In this thesis, gold nanoshells were successfully designed to target breast cancer cells through antibodies against the extracellular Human Epidermal growth factor Receptor 2 (HER2) , whose overexpression is associated with more aggressive forms of breast cancer. By comparing HER2-positive breast cancer cells to normal (nonneoplastic) breast cells, the nanoshells effectively labeled HER2- overexpression within 5 minutes of incubation time. These nanoshells also enhanced contrast of the same cancer cells using two-photon microscopy, which enabled subsequent validation of preferential labeling using a distinctive co-culture experiment. To ultimately translate these findings to the clinic, nanoshells of similar design were studied for their effectiveness at enhancing contrast of malignancy in breast tissue sections and intact human breast tissue. Through detailed experimental conditions, these nanoshells increased contrast of cancer cells in sectioned HER2-overexpressing breast tissue within 5 minutes of incubation time using reflectance confocal microscopy, a unique imaging capability not previously reported. Finally, these targeted nanoshells ii were used to effectively visualize HER2 receptor expression in intact human breast tissue specimens within the same 5 minute incubation time point. Through two-photon imaging, it was shown that these nanoparticies preferentially labeled tissue surface receptors, with minimal penetration depth. Importantly, the enhanced surface labeling was observed macroscopically through a standard stereomicroscope and confirmed microscopically through reflectance confocal microscopy and immunohistochemistry. These results suggest that anti-HER2-nanoshells used in tandem with a near-infrared reflectance confocal microscope and a standard stereomicroscope may potentially be used to discern HER2-overexpressing cancerous tissue from normal tissue in near real time and offer a rapid supplement to current diagnostic techniques. 111 were used to effectively visualize HER2 receptor expression in intact human breast tissue specimens within the same 5 minute incubation time point. Through two-photon imaging, it was shown that these nanoparticles preferentially labeled tissue surface receptors, with minimal penetration depth. Importantly, the enhanced surface labeling was observed macroscopically through a standard stereomicroscope and confirmed microscopically through reflectance confocal microscopy and immunohistochemistry. These results suggest that anti-HER2-nanoshells used in tandem with a near-infrared reflectance confocal microscope and a standard stereomicroscope may potentially be used to discern HER2-overexpressing cancerous tissue from normal tissue in near real time and offer a rapid supplement to current diagnostic techniques. iii