Lymphedema represents one of the major problem of morbidity in breast cancer therapy. Approximately 15-30% of patients show more or less severe lymphedema of the arm, following cancer therapy. Main pathogenetic mechanisms, risk factors, main grading criteria and scales as LENT-SOMA, CTCv2, CTCAE v3 are presented. A close correlation has been documented between the extent of axillary dissection and the association with radiotherapy in determining an increased risk of lymphedema. Details of surgery and radiotherapy are relevant in the definition of the risk of edema of the arm. Because the axillary area does not correspond to an organ, evaluable parameters as V20 and Dmean available for other organs are not applicable. There is some evidence of a correlation between the irradiation volume and the development of lymphedema. Data of the impact of the dose and its fractionation on the development of lymphedema are contrasting. The monitoring system of late toxicity used by the authors is presented.