HER2-targeted therapies have greatly improved the outcome for patients with HER2-positive breast cancer. However, resistance to these therapies is an on-going clinical problem. Therefore, novel therapeutic strategies to overcome or prevent resistance are required. The aim of this PhD project was to investigate mechanisms of resistance to HER2-targeted therapies and to develop strategies to overcome resistance. Two lapatinib resistant cell lines (SKBR3-L and HCC1954-L) previously generated in our lab showed increased protein phosphatase 2A (PP2A) activity. In this study, the resistant cell lines were more sensitive to PP2A inhibition by okadaic acid and the therapeutic PP2A inhibitor LB-100. PP2A inhibition also enhanced the effect of lapatinib and the combination induced apoptosis in both cell lines. Addition of LB100 to lapatinib prevented the development of lapatinib resistance in two lapatinib-naive cell lines. HCC1954-L cells produced xenograft tumours in mice. The combination of lapatinib and LB-100 did not cause systemic side effects, deeming the combination safe for efficacy testing in the HCC1954-L xenograft model. In a panel of HER2-positive cell lines, sensitivity to PP2A inhibition with okadaic acid correlated with lapatinib resistance. PP2A catalytic subunit and structural subunit expression did not correlate with lapatinib or PP2A inhibition response. However, expression levels of the PP2A inhibitor CIP2A correlated with improved survival in HER2-positive breast cancer patients in a publicly available dataset. An afatinib-resistant SKBR3 cell line (SKBR3-A) was analysed by reverse phase protein array. Phospho-Src (Y416) was elevated in SKBR3-A compared to SKBR3 cells. SKBR3-A cells were more sensitive to Src inhibition by dasatinib and the combination of afatinib and dasatinib was highly synergistic. The combination inhibited both HER2/EGFR and Src signalling and caused non-apoptotic cell death. Addition of dasatinib prevented the development of afatinib resistance in three of four treatment-naive HER2-positive cell lines and two of three cell lines with acquired trastuzumab resistance. In conclusion, the drug combinations of lapatinib plus LB-100 and afatinib plus dasatinib show potential for the treatment of HER2-positive breast cancer.