There are many similarities in tumour development between plants and animals, but fundamental differences prevent plants from developing cancer. In particular, cell division and proliferation are strictly regulated in plants, and plant tumours cannot metastasize owing to the rigid microenvironment surrounding plant cells (the cell wall). What can tumour development in plants tell us about cancer in animals? In plants, as in animals, most cells that constitute the organism limit their reproductive potential in order to provide collective support for the immortal germ line. And, as in animals, the mechanisms that restrict the proliferation of somatic cells in plants can fail, leading to tumours. There are intriguing similarities in tumorigenesis between plants and animals, including the involvement of the retinoblastoma pathway as well as overlap with mechanisms that are used for stem cell maintenance. However, plant tumours are less frequent and are not as lethal as those in animals. We argue that fundamental differences between plant and animal development make it much more difficult for individual plant cells to escape communal controls.