The various types of physical limitations on the miniaturization of active elements are classified. Those physical limitations on the sizes of elements which arise in the manufacture of integrated circuits are analyzed. The various devices are classified by size. A scaling model which describes the changes in the parameters of the devices in the course of miniaturization is discussed. The limitations of scaling are discussed. Physical limitations which are set on the sizes of elements by their operating mechanism are examined. The most important limitation on the degree of integration of elements on a chip and on the working speed--the product of the number of elements and the reciprocal of their switching time--is imposed by the heating of the chip. It is concluded that the physical limitations are unimportant down to dimensions of the order of 0.1 $\mu$m; that in order to reduce the size of elements below 0.1$\mu$m it is necessary to produce elements which have no p-n junctions and which are heterostructures; and that the change in conduction mechanism in the structures with dimensions less than 0.1$\mu$m will require the development of new types of elements: ballistic and tunnel transistors.