This volume contains the proceedings of the AMS Special Session on Representation Theory and Flag Varieties, held at the AMS Sectional Meeting at SUNY Buffalo on September 9-10, 2023. Representation theory, which originated with Frobenius's work on group representations in the late 19th century, explores symmetries in mathematics and natural sciences. It has evolved into a broad field since then. Its geometric approach, leverages flag and Nakajima quiver varieties, produces significant results such as the resolution of the Kazhdan-Lusztig conjecture, connects to disciplines such as enumerative geometry, algebraic geometry, and mathematical physics, and in return reveals hidden structures within these varieties. This volume compiles recent advancements in representation theory and flag/quiver variety geometry, offering original research and expository articles. Contributions include work on generalized Shubert calculus, quantum $K$-theory of semi-infinite flag variety, research on geometry of Springer fibers and their variants, combinatorial model for quiver varieties, and results on $G$-connections in the geometric Langlands program, providing fresh insights and methodologies.