Unified field theories were a topic Einstein worked on. He never succeeded in finding a unified field theory. To find such a theory, you have to (1) write down a conjectured field equation; (2) find various solutions to the equation, each representing a physical situation; and (3) see if they match up with experiment and make new predictions that can be tested. When we say that he didn't succeed, we mean that he did #1 and #2, but none of his attempts passed the test in #3.
-Ben