In the science of the theory of state and law, a problem related to the classification of forms of state structure has arisen. The traditional approach to the form of state structure, within which it is customary to distinguish unified (unitary) and constituent (federal) states, does not reflect contemporary life realities. There are no clear criteria in legal science for dividing states into unified and constituent ones. The paper considers various approaches to the problem of classifying forms of state structure in contemporary legal theory. The author concludes that the traditional three-element approach to the form of state (form of government, form of organization and political regime) has methodologically exhausted itself and does not reflect the current state of affairs. All elements of the state form are closely interrelated and are in an unbreakable dialectical unity. The form of government largely determines the form of the territorial structure, as well as the methods of interaction between the authorities and the population (political regime). The nature of the relationship between the center and the regions (constituent entities) based on the centralization or decentralization principle, largely depends on the form of government. As a rule, the republican form of government determines the federal (decentralized) form of state-territorial structure and the democratic political regime. At the same time, the real possibility of functioning of republican institutions (election and alternation of government authorities) and the implementation of the federalism principles (the actual independence of the constituent parts of the state) largely depend on the political regime. In the context of an authoritarian (totalitarian) political regime, the republican form of government and the federal form of government enshrined in legislation are conditional in nature – in fact, they cannot be implemented. The purpose of the work is to analyze the forms of state structure in contemporary legal theory and identify the essential features of empire as a special form of state structure.