Here we use intravital imaging to demonstrate a reversible transition to a motile state as breast cancer cells spread. Imaging primary tumours revealed heterogeneity in cell morphology and motility. Two distinct modes of motility were observed: collective and single-celled. By monitoring the localization of Smad2 and the activity of a TGFβ-dependent reporter gene during breast cancer cell dissemination, we demonstrate that TGFβ signalling is transiently and locally activated in motile single cells. TGFβ1 switches cells from cohesive to single cell motility through a transcriptional program involving Smad4, EGFR, Nedd9, M-RIP, FARP and RhoC. Blockade of TGFβ signalling prevented cells moving singly in vivo but did not inhibit cells moving collectively. Cells restricted to collective invasion were capable of lymphatic invasion but not blood-borne metastasis. Constitutive TGFβ signalling promoted single cell motility and …