When normal cells meet, in vitro at least, they politely step aside. This process, known as contact inhibition of locomotion, was first recognised more than 50 years ago and involves the cells retracting their protrusions and changing direction on contact. Malignant invasion has been attributed to failure to conform to this orderly conduct, but there has, so far, been no evidence that cells behave this way in vivo. Now a study published in the journal Nature describes the behaviour of neural crest cells, a highly migratory and multipotent embryonic cell population. When two migrating neural crest cells meet, either in vitro or in vivo, they stop, collapse their protrusions and change direction. By contrast, if a neural crest cell encounters a different cell type, it fails to demonstrate contact inhibition of locomotion and, instead, invades the other tissue in a way reminiscent of metastatic cancer cells. The authors further showed that inhibition of non-canonical Wnt-signalling abolished both contact inhibition of locomotion and the directionality of neural crest migration. The demonstration of contact inhibition of locomotion in vivo and elucidation of an underlying pathway may lead to new ways to prevent tumour metastasis.