The ontogenetic theory of locoregional cancer spread regards cancer as a clinical manifestation of the pathological reactivation and maintenance of the sequential developmental programmes that previously controlled the stepwise embryological morphogenesis of the tissue from which the cancer originated. In the state of morphostasis that characterises adult organisms, these programmes are silenced. During malignant progression, these programmes run in retrograde sequence, which leads to cancer infiltration of ever larger tissue areas. However, because the reactivated morphogenetic programmes need topologically defined tissue domains—morphogenetic fields—to provide positional information for their interpretation, local tumour propagation is confined to permissive compartments (topographically defined tissue domains where malignant cells can survive, migrate, and proliferate), which are determined by the state of malignant progression. The tissue at risk of local tumour spread, the cancer field, is the mature tissue derived from the corresponding morphogenetic field in the embryo, which is labelled with the respective positional information. The theory can be tested morphologically and clinically for all tumours. Verification of this theory would offer substantial potential to improve prognostic assessment and surgical treatment. Identification of the complementary positional information for tumour cells in different ontogenetic stages, and their associated cancer fields, could be a molecular research strategy to further test the theory.