An investigation of the structure of carbons of different origin treated at temperatures between 1000 and 3000 degrees C has shown that the graphitizing and non-graphitizing carbons form two distinct and well-defined classes. The differences in structure are apparent from the earliest stages of carbonization, and may be attributed mainly to the formation at low temperatures, in the non-graphitizing carbons, of a strong system of cross-linking uniting the crystallites. This leads to a random orientation of the crystallites in a rigid, finely porous mass. In the graphitizing carbons the cross-linking is much weaker, the structure is more compact, and neighbouring crystallites have a strong tendency to lie in nearly parallel orientation. It is shown that crystallite growth occurs by the gradual displacement of whole layer-planes or even of groups of layer-planes. The pre-orientation existing in the graphitizing carbons facilitates this process, enabling the rearrangement of the layer-planes to take place by small stages. It is the principal factor favouring crystallite growth in the graphitizing carbons. In the non-graphitizing carbons crystallite growth is impeded both by the strong cross-linking between neighbouring crystallites and by their random orientation.