Let E be an elliptic curve whose equation has integer coefficients, let N be the so-called j-conductor of E and, for each n, let a_n be the number appearing in the L-function of E. Then, in technical terms, the Taniyama-Shimura conjecture states that there exists a modular form of weight two and level N which is an eigenform under the Hecke operators and has a Fourier series suma_nq^n.