Background: Neoadjuvant chemotherapy (NACT) results in eradication of cancer in the axillary nodes in 40% to 70% of patients. This raises questions about the benefit of further axillary treatment in those patients with no evidence of residual nodal disease (ypN0) after NACT. Methods:Design: ATNEC is a phase 3, randomised (1:1), multi-centre trial, with embedded economic evaluation, comparing standard axillary treatment (axillary lymph node dissection [ALND] or axillary radiotherapy [ART]) with no further axillary treatment in T1-3N1M0 breast cancer patients with needle biopsy proven axillary nodal metastases, who after NACT have no residual nodal disease (ypN0) on dual tracer sentinel node biopsy (SNB) and removal of at least 3 nodes (sentinel nodes and marked involved node). Stratification: Institution, type of surgery (breast conserving surgery vs mastectomy), receptor status (triple negative vs HER2 positive vs ER positive and/or PR positive and HER2 negative). Inclusion criteria are: Age ≥ 18, Male or female, T1-3N1M0 breast cancer at diagnosis (pre-NACT), FNA or core biopsy confirmed axillary nodal metastases at presentation, ER and HER2 status evaluated on primary tumour, received standard NACT as per local guidelines, ultrasound of the axilla at completion of NACT, dual tracer SNB after NACT and at least 3 nodes removed (sentinel nodes and marked node), no evidence of nodal metastases post NACT (ypN0). Exclusion criteria are: bilateral invasive breast cancer, SNB prior to NACT, marked node not removed except where at least one node removed shows evidence of down-staging with complete pathological response e.g. fibrosis/scarring and at least 3 nodes removed, previous ipsilateral axillary surgery, previous cancer within last 5 years or concomitant malignancy except basal or squamous cell carcinoma of the skin, in situ carcinoma of the cervix, in situ or stage 1 melanoma, contra- or ipsilateral in situ breast cancer. Aims: To assess whether, omitting further axillary treatment (ALND and ART) for patients with early stage breast cancer and axillary nodal metastases on needle biopsy - who after NACT have no residual nodal disease on SNB (ypN0) - is non-inferior to axillary treatment in terms of disease free survival, and reduces the risk of lymphoedema at 5 years. Statistical methods: All analyses will be carried out on an intention-to-treat basis to preserve randomisation, avoid bias from exclusions and preserve statistical power. Radiotherapy quality assurance: Study has in-built radiotherapy QA programme that will be co-ordinated by National Radiotherapy Trials QA (RTTQA) group. Target accrual: 1900. Trial status: Recruiting. Number of sites: 100. Clinical trial information: NCT04109079