<p>High-entropy materials (HEMs), encompassing high-entropy alloys, oxides, carbides, nitrides, borides, and other compositionally complex systems, have emerged as a transformative class of materials owing to their exceptional compositional flexibility, tunable electronic structures, and multi-functional properties. The incorporation of multiple principal elements generates diverse local chemical environments, enhancing structural stability, surface reactivity, and functional performance. This review discusses the fundamental principles of HEMs, focusing on thermodynamic stabilization, phase formation, electronic structure, microstructure evolution, and their processing–microstructure–property relationships. Particular attention is devoted to how synthesis strategies and processing parameters regulate phase constitution, grain size, local chemical ordering, defects, and interfaces, thereby determining the electronic, catalytic, electrochemical, and sensing properties of HEMs. Recent advances in scalable synthesis routes, including solid-state, wet-chemical, vapor-phase, electrochemical, and emerging non-equilibrium approaches, are critically discussed alongside their influence on microstructural evolution and application-specific performance. The review highlights the rapidly expanding applications of HEMs in heterogeneous catalysis, gas sensing, hydrogen production and storage, batteries, CO₂ conversion, thermoelectric, photovoltaics, and biofuel-related technologies, with an emphasis on the underlying mechanisms governing their superior performance. By integrating recent advances across synthesis, microstructure engineering, and multi-functional applications, this review establishes a framework for the rational design of HEMs and outlines future directions for their translation from laboratory-scale materials to practical technologies in sustainable energy, environmental remediation, and advanced electronics.</p>