Re-irradiation is probably the most challenging situation in radiotherapy because the radiation tolerance of the normal tissue is significantly reduced compared with the first treatment series. Results with traditional radiotherapy techniques have been disappointing because of the poor conformality of the dose distributions: radiation doses were either insufficiently low resulting in poor rates of tumor control or substantial toxicity was the consequence of high-dose re-irradiation. This chapter will focus on modern techniques of radiation treatment planning and delivery, which make improved sparing of the normal tissue possible. All techniques will be discussed in the context of re-irradiation and theoretical and clinical data supporting the use of these technologies will be presented. Palliative reirradiation to moderate doses might be feasible without using advanced technology. However, under many circumstances 2D or 3D conformal approaches cannot fulfill the required normal tissue constraints. The present chapter discusses the advantages and challenges associated with more complex planning and delivery methods.