The processes of electrical spin injection, transport, and detection in nonmagnetic semiconductors are inherently nonequilibrium phenomena. A nonequilibrium spin polarization, which we will refer to in this chapter as spin accumulation, may be generated and probed by a variety of optical and transport techniques. We will focus on spin accumulation in heterostructures of ferromagnetic metals (particularly Fe) and III–V semiconductors (particularly GaAs). Much of the essential physics of these systems for both metals and semiconductors has been addressed in other chapters in this volume. The basic phenomena of spin accumulation in metals have been covered in Chapter 5, Volume 1 and Chapter 9, Volume 3. This chapter will provide a review of a sequence of experiments that have advanced the understanding of spin transport in semiconductors through their sensitivity to electron spin dynamics. To a great extent, this has been a matter of careful experimental design and execution, and a good part of the discussion will address tests of well-known models. The benefit of this approach has been a reliable demonstration of all-electrical devices incorporating a ferromagnetic (FM) injector, semiconductor (SC) channel, and FM detector. In this context, we will show how quantitative electrical spin detection measurements can be made with these devices using both optical imaging and transport techniques, and we will conclude with some discussion of the new physics that can be addressed. Although the emphasis of the chapter is on the progression of experiments leading up to the demonstration of a III–V based lateral spin valve, some discussion of more recent developments as well as additional references are included in this revision.