The light-emitting electrochemical cell (LEC) can conveniently and broadly be defined as a device that generates light with reversible electrochemical means within a material that allows for both electron and ion transport. The concept of employing such electrochemical cells for the generation and study of light-emitting processes was conceived and demonstrated in the 1960s1-4 and the field has thereafter flourished and expanded, as evidenced, for example, by a number of excellent reviews on the topic.5-12 The main focus of this chapter is on relatively recent high-performance solid-state LECs, which have begun to attract commercial interest in specialty lighting and display applications,13 but we find it appropriate to begin in Section 5.2 with a historical overview of the research work in related LEC fields that laid the groundwork for today’s state-of-the-art devices. Thereafter, in Section 5.3, the main types of solid-state LEC devices are introduced, and in Section 5.4 the complex and intensely debated operational mechanism of such solid-state LECs is discussed. In Sections 5.5 and 5.6, a number of key parameters of solid-state LECs in the context of usefulness in applications-the turn-on time, the power conversion efficiency, and the operational lifetime-are reviewed. The chapter ends with a conclusion in Section 5.7, where the opportunities and challenges for these fascinating devices in the future are highlighted.