Despite the rapidly expanding ambit of physical research and the continual appearance of new branches of physics, the main thrust in its development was and is the attempt at a theoretical synthesis of the entire body of physical knowledge. The main triumphs in physical science were, as a rule, associ­ ated with the various phases of this synthesis. The most radical expression of this tendency is the program of construction of a unified physical theory. After Maxwellian electrodynamics had unified the phenomena of electricity, magnetism, and optics in a single theoretical scheme on the basis of the con­ cept of the electromagnetic field, the hope arose that the field concept would become the precise foundation of a new unified theory of the physical world. The limitations of an electromagnetic-field conception of physics, however, already had become clear in the first decade of the 20th century. The concept of a classical field was developed significantly in the general theory of relativity, which arose in the elaboration of a relativistic theory of gravitation. It was found that the gravitational field possesses, in addition to the properties inherent in the electromagnetic field, the important feature that it expresses the metric structure of the space-time continuum. This resulted in the following generalization of the program of a field synthesis of physics: The unified field representing gravitation and electromagnetism must also describe the geometry of space-time.