Tumour development comprises a complex succession of events that prompt incipient cancer cells to proliferate out of control and to acquire migratory and invasive capabilities. Over the past decades, cancer research has produced a wealth of knowledge about the intrinsic alterations of neoplastic cells within tumours. However, tumours now have come to be understood to function as complex tissues in which numerous cells, collectively termed the tumour microenvironment, play a critical role. These include inflammatory cells, blood vessels, fibroblasts and changes in their secreted extracellular matrix. This is revealing additional mechanisms of tumorigenesis that has spawned a new era of therapeutic strategies aimed at eradicating tumours.