[en] Improved local control rates have been demonstrated in retrospective studies as well as in randomized trials on brachytherapy with increasing doses to the tumour bed. The higher local control obtained by interstitial breast implants, as compared to external photon or electron beam boosts, have been mainly attributed to the higher doses actually delivered to the tumour bed by these implants for the same nominal dose as compared to external beam radiotherapy (RT). On the other hand, poor cosmesis has also been correlated with radiation dose to the breast skin (radiation telangiectases), and breast tissue (retraction due to fibrosis), the latter depending not only on RT dose but also on the treated boost volume. For this reason, a possible benefit of interstitial implants will only be realized when the gain in local control goes together with minimal cosmetic damage. Therefore, the ballistic advantages of interstitial implants have to be maximally exploited: i.e. the treated volume should be maximally adapted to the target volume, and additional irradiation of the breast skin by the boost technique should be avoided. This paper deals in detail with the technical aspects of breast brachytherapy that seem to be relevant for high quality outcome. (author)