In the recent years, Cloud Computing has been one of the top ten new technologies which provides various services such as software, platform and infrastructure for internet users. The Cloud Computing is a promising IT paradigm which enables the Internet evolution into a global market of collaborating services. In order to provide better services for cloud customers, cloud providers need services that are in cooperation with other services. Therefore, Cloud Computing semantic interoperability plays a key role in Cloud Computing services. In this paper, we address interoperability issues in Cloud Computing environments. After a description of Cloud Computing interoperability from different aspects and references, we describe two architectures of cloud service interoperability. Architecturally, we classify existing interoperability challenges and we describe them. Moreover, we use these aspects to discuss and compare several interoperability approaches.