The nature of neoplasia and its sometime end result, cancer, has been studied by exposition and explanation of the sequential lesions of tumour progression. Neoplastic lesions were divided into four classes on the basis of growth characteristics and whether lesional growth is confined to one or more tissue compartments. Class IA, the initial lesion, an orderly, probably clonal growth, usually differentiates and disappears. Class IB: Failure to differentiate accompanied by disorderly growth. Class IC: Randomly dispersed atypical cells, constituting a precursor state. Class II, intermediate lesions, apparently arising from the atypical cells, show temporally unrestricted growth within the tissue compartment of origin. Class III lesions, primary invasive cancers, show temporally unrestricted growth in two or more tissue compartments and metastasise along different paths, a property associated with extracellular matrix interaction. The metastatic pathways may result from different subsets of cells in the primary cancer. Class IV lesions are the metastases. It was concluded that, all neoplasms develop in the same way, have the same general behavioural characteristics, and, when malignant, all interact with the extracellular matrix of the primary and the secondary sites. The origins and development of cancer are considered to be pluralistic and not due to a discrete change in a cell, whose progeny, as a result of that discrete change, carries all of the information required to explain the almost limitless events of a neoplastic system.