The article discusses Albert Einstein’s unique ability to devise, pursue, and exploit imaginary physical situations: his dreams. Although such thought- or gedanken-experiments were always based on commonly held premises, Einstein was able, over and over, to use gedankenexperiments to capture the barest physical essentials of a situation, and to proceed from those essentials to their inescapable consequences, no matter how astonishing, no matter how remote from previous conventional wisdom. The paper describes and discusses the thought-experiments that Einstein used in achieving (...) his most notable successes, the Special and General Theories of Relativity. It discusses the origins of each, their physical meaning, and some of the ways in which the principles embodied in the thought-experiments contributed to his subsequent discoveries. The paper concludes by contrasting Einstein’s Special and General Relativity campaigns with his search for a theory describing gravity and electromagnetism as part of a greater whole, a Unified Field Theory. In almost thirty years of trying, he made little or no progress. Notably, Einstein never reported devising a thought-experiment to motivate or guide him in this, the longest effort of his career. (shrink)