The focus of this paper is to formalize the concept of identity-based threshold signature and give the first provably secure scheme based on the bilinear pairings. An important feature of our scheme is that a private key associated with an identity rather than a master key of the Public Key Generator is shared among signature generation servers, which is more desirable in practice. Another interesting aspect of our results is that the security of one of the verifiable secret sharing schemes used to construct the identity-based threshold signature scheme is relative to a slightly modified version of the Generalized Tate Inversion problem recently proposed by Joux.