Regardless of the enormous investments in cancer research and drug development, the proportion of approved drugs in oncology is low compared to other indications, and new avenues are needed. One attractive approach in this regard is drug repositioning where new uses outside the scope of the original medical indications for existing drugs are identified. It offers the advantages of reduced development risks, time and cost over de novo drug discovery pathways. The main focus of this thesis was to explore and employ different strategies to identify repurposable drug candidates for treatment of cancer. Aiming for this, in the first project we followed a bioinformatics approach to evaluate PDE3A as a drug target and biomarker. We showed that subgroups of tumors, within many different cancer types, overexpress PDE3A (mRNA and protein) and that PDE3A can predict sensitivity to the clinically tested phosphodiesterase inhibitors zardaverine and quazinone (Paper I). In the second project, a novel automated image based microscopy assay was developed and used for detection of apoptotic cells. In a screen the method was successfully used to identify apoptosis inducing compounds. Two of these apoptosis inducers were found to have repurposing potential (Paper II). Moreover, high-throughput combination screening was performed using different cell models. In paper III, monolayer cell cultures were used as cell model to search for combination partners for the anti-parasitic compound mebendazole (a repurposing candidate). As a result, the antipsychotic drug thioridazine was found to have synergistic effect when combined with mebendazol. Finally, a novel drug-combination platform for three-dimensional cell culture based screening, in 384 well formats, was developed. This assay was used to search for combination partners to the anti-parasitic compound nitazoxanide (a repurposing candidate), which was previously reported to specifically target quiescent cancer cells. The screen identified the antifungal agent ketoconazole as selectively toxic to hypoxic and nutrient deprived cancer cells when combined with nitazoxanide (Paper IV). Thus, we have developed/explored several methodological approaches and identified drugs that potentially can be repurposed for treatment of cancer.