Adoptive cell therapy (ACT) by infusion of autologous or redirected tumor-specific T-cells has had a major impact on the treatment of several metastasized malignancies that were until now hardly treatable. Recent findings provide a more profound knowledge on the underlying mechanisms of success and allow the optimization of the ACT protocol with respect to (1) the treatment related side-effects, (2) the quality and specificity of infused T-cells, and (3) the immunosuppressive phenotype of the tumor environment. In this review, the results and insights in the success of ACT as well as the possibilities to improve ACT and its exploitation as treatment option for various metastatic cancer types, will be discussed.