Public Key Infrastructure and Certification Authorities (PKI/CA) have been used to support secure unclassified transactions over open networks, thus it promotes e-commerce, e-government, and electronic transactions protection. But the problem of PKI interoperability caused by the difference of the realization of technique and certificate policy has restricted the application of PKI. This article introduces the technique of Cross-Certification and its realization models. The emergency of Cross-Certification research and application landscape are described also. Furthermore, this article discusses several key problems those should be solved when preparing to carry the Cross-Certification model into practice.