Two-dimensional (2D) electrochromic materials are functional materials that modify optical properties in response to electrical impulses. This chapter delves into an overview of the fundamental mechanisms, classes of materials, and applications with a particular focus on optoelectronic devices. 2D systems such as graphene derivatives, transition metal dichalcogenides (TMDs), MXenes and black phosphorus are advantageous as compared to the conventional bulk materials. They offer advantages such as tunable band structures, high surface areas, ultrathin form factors and rapid ion diffusion pathways, proving them ideal for reversible, fast and spectrally diverse optical modulation. The working principles for electrochromism, such as ion intercalation, redox-induced bandgap shifts, and phase transitions, are explored in these materials. Further, the effect of these mechanisms on efficient control over light transmission, reflection, and absorption across visible and infrared regimes is examined. The chapter categorises 2D electrochromic materials and compares their optical contrast, operational voltage, switching speed and stability. Moreover, the fabrication strategies, device architectures suited for flexible displays, smart windows, optical modulators, and dual-function electrochromic energy systems are detailed. A section is dedicated to bridge the gap between materials chemistry and device functionality. Finally, the major challenges in scalability, environmental stability and integration are highlighted. Thus, this chapter will serve as a foundation for engineers and researchers working on next-generation 2D electrochromic materials and devices.