Tonghai Yang specializes in number theory, arithmetic geometry, and automorphic representation, and in particular their interaction. He studies L-functions of elliptic curves and abelian varieties using tools from representation theory and then obtains its arithmetic consequences. An L-function provides a beautiful bridge between the arithmetic of a geometric object (say elliptic curve) and the local information one can get from this object. Tonghai is also exploring some connection between Eisenstein series (some explicit modular forms) and arithmetic on Shimura curves.