HE evolution of the aerospace technology is marked by a series of technology achievements across the spectrum of the engineering disciplines, and one of the more interesting stories in the history of the aerospace industry relates to the development of electrical power systems for aircraft and spacecraft. The research and development effort leading to today’ s systems has spanned 10 decades and has continued through world wars and economic depressions. It was dependent on a marriage of ideas from virtually all e elds of engineering: electrical, mechanical, materials, industrial, chemical, computer, and aerospace. It has achieved a remarkable state of technology even though it was never as prominent as the developments in propulsion systems, as elegant as breakthroughs in avionics, or as intensely studied as structures and aerodynamics. It has always been the “ other” system, one that everyone assumed would be there at the appropriate time. And it was. The evolution of aerospace electrical power systems from the early days of powered e ight through today is marked by a combination of serendipity, insight, innovation, and hard work. In this paper we will recall some of the early dife culties faced by aircraft designers, the processes through which they addressed the challenges, and how they went about creating the technology breakthroughs leading to today’ s aircraft and spacecraft electrical systems. At various points in the review, we pause to probe more deeply into the key technologies of that day to give the reader a better appreciation of the ingenuity of the early engineers who created this technology. Such a review could not hope to cover all of the topics of relevance or of interest, and so a subjective selection has been imposed, hopefully one that does not do too great an injustice to aerospace power history or to those who created it. The story is presented in two parts: e rst, the development of aircraft systems and, then, spacecraft systems. This is done, as the reader will see, because of the differences in the constraints, operational environments, and demands on the two systems. In one important aspect, however, they do share at least one common trait: not surprisingly, each benee ted from an opportunity to harvest existing technologies until the demand for power exceeded the ability