First published in1966, here is presented a comprehensive overview of one of the most elusive scientific speculations by the pre-eminent genius of the 20th century. The theory is viewed by some scientists with deep suspicion, by others with optimism, but all agree that it represents an extreme challenge. As the author herself affirms, this work is not intended to be a complete treatise or 'didactic exposition' of the theory of unified fields, but rather a tool for further study, both by students and professional physicists. Dealing with all the major areas of research which together comprise the development of a working model, the author ranges over conservation equations, variational principles, solutions of spherical symmetry, and treats a wide selection of Einstein's own equations. The final chapter indicates problems associated with the unified field theory, in particular the energy-momentum tensor and geodesics.