Durotaxis, the process by which cells follow gradients of extracellular mechanical stiffness, has been proposed as a mechanism driving directed migration. Despite the lack of evidence for its existence in vivo , durotaxis has become an active field of research, focusing on the mechanism by which cells respond to mechanical stimuli from the environment. In this review, we describe the technical and conceptual advances in the study of durotaxis in vitro , discuss to what extent the evidence suggests durotaxis may occur in vivo , and emphasize the urgent need for in vivo demonstration of durotaxis. Durotaxis is the process by which cell movements are coordinated by environmental stiffness gradients. In this review, Shellard and Mayor describe recent advances in our understanding of durotaxis in vitro and the challenges to study it in vivo. [ABSTRACT FROM AUTHOR]