ErbB2 is one of the four members of the epidermal growth factor family of receptor tyrosine kinases, which mediate the activation of several signaling pathways that lead to cell proliferation, survival, differentiation and migration. ErbB2 is the only member of the family that does not have a known ligand, but it is the preferred dimerization partner of all the other members of the family. This receptor is overexpressed in up to 30% of all breast cancers and is resistant to down-regulation. ErbB2 is stabilized by the chaperone Hsp90 and this thesis confirms that the Hsp90 inhibitor 17-AAG leads to ErbB2 internalization and degradation in MDA-MB-453 cells. The study also confirms that PMA-mediated activation of PKC induces ErbB2 internalization. However, unlike 17-AAG, PMA does not induce degradation of ErbB2. MDA-MB-453 cells were chosen because they have been reported to overexpress extracellular Hsp90 (eHsp90). Hsp90 was originally believed to function only intracellularly, but recent data show that Hsp90 under conditions like stress, can be secreted. Hsp90 is secreted by a large number of cancer cells and eHsp90 has been shown to be involved in cancer invasiveness. It has also been suggested that eHsp90 is the reason why cancer cells are more vulnerable to Hsp90 inhibitors. One of the aims of this thesis was thus to initiate studies on how eHsp90 affects ErbB2 and how eHsp90 can be targeted. Therapeutic antibodies that specifically target eHsp90 could lead to the development of future drugs that are less harmful to normal cells, and some experiments involving anti-Hsp90 antibodies were performed. ErbB2 is targeted by therapeutic antibodies like Herceptin (trastuzumab) and its derivative Kadcyla (trastuzumab emtansine). The cytotoxic effect of Kadcyla depends on its internalization and degradation. This thesis shows, using Herceptin as a model, that both 17-AAG and PMA induces internalization of the antibody, but as for ErbB2, only 17-AAG induces degradation. Finally, since eHsp90 has been shown to interact with the extracellular domain of ErbB2, it may affect the binding of therapeutic anti-ErbB2 antibodies. With the assumption that anti-Hsp90 antibodies can have a neutralizing effect, it was thus studied whether such antibodies affect binding of Herceptin. Under the conditions tested no clear effect was observed. This might, however, depend on antibody concentrations and overall the experiments in this thesis have made the fundament for further studies of eHsp90.