Cloud computing is a relatively new paradigm that promises to revolutionize the way IT services are provided. There are multiple benefits that companies can gain from cloud computing. However, there still remains a number of issues to be solved before this new computing paradigm is widely adopted. In this paper we introduce the issues of portability and interoperability and we focus on the cross-platform portability of applications. We present some high level approaches and existing works that try to address this issue. Finally we briefly propose some future research directions towards investigating how to improve the portability of applications across cloud platforms.