Using high-resolution multiphoton microscopy of mammary carcinomas in mice, Gligorijevic et al. assessed tumour cell phenotype and the likelihood of metastasis. Slow-moving tumour cells had invadopodia, whereas fast-moving cells did not, and both types of locomotion were present in different regions of the same tumour. Further analyses revealed that only tumour cells in invadopodium-rich environments were able to degrade the extracellular matrix (ECM) and disseminate, and the number of invadopodia correlated with ECM degradation. Inhibiting metalloproteinases prevented ECM degradation and metastasis to the lung. Further understanding of how motile phenotypes correlate with metastasis may improve treatment options for patients.