D O C . 4 5 9 F I E L D T H E O R Y & H A M I LT O N P R I N C I P L E 4 0 9 459. “Unified Field Theory and Hamilton’s Principle” [Einstein 1929dd] Presented 21 March 1929 Published 23 April 1929 In: Preußische Akademie der Wissenschaften (Berlin). Physikalisch-mathematische Klasse. Sitzungsberichte (1929): 156–159. In a contribution to these Reports that recently appeared [1929, I],[1] I formulat- ed field equations for a unified field theory without basing their derivation on a variation principle. The justification of those field equations rests on the presump- tion of the compatibility of the 16 field equations (10),[2] loc. cit. Since I was not able to establish four identity relations among these equations, Mr. Lanczos and Mr. Müntz have expressed well-founded doubts as to their admissibility, without a clear conclusion to the matter having been reached thus far.[3] In the meantime, I have found that it is possible to solve this problem in a quite satisfactory manner by making use of Hamilton’s principle, whereby the mutual compatibility of the equations is guaranteed from the beginning. The identities derived in my previous work as well as the notation employed there will be used here, or adopted. [p. 156]