This article treats the current state of fundamental understanding, development, and applications of alloys, with emphasis on alloy chemistry rather than physical metallurgy. First, alloy phases and phase diagrams are treated from a computational viewpoint. Next, technical alloys, including steels, aluminum alloys, magnesium alloys, superalloys, and titanium alloys, are reviewed; structural intermetallics are treated as a paradigm for theoretical predictive methods. A review of important specialty alloys follows, including hard and soft magnetic alloys, superconductors, shape memory, and magnetostrictive alloys, hydrogen storage alloys, thermoelectric alloys, solders, and alloy coatings. A brief survey of current advanced alloy preparation and characterization methods concludes the article.