Blockchain is one of the most recent disrupting technologies. Since Bitcoin emerged as its initial application, many blockchain projects arose offering different features for general and domain specific scenarios. In turn, Blockchain platforms work as isolated environments and they do not support interaction (i.e. interoperability) between each other.During the last years, some academic and industry efforts were performed in order to achieve interoperability between blockchain platforms. However, these efforts solve specific use cases and do not provide a general interoperability solution. In order to organize existing work in the area, this paper provides a feature-based classification framework based on 16 key interoperability features, that enabled the classification of 65 academic papers in three categories and six subcategories.The results show that nowadays research is focused on public interoperability, while private and public-to-private interoperability are still on their first steps. Based on these results, this work also discusses nowadays blockchain interoperability landscape, identifies challenges, and suggests future research directions.