  Blockchain interoperability is emerging as one of the crucial features of
blockchain technology, but the knowledge necessary for achieving it is
fragmented. This fact makes it challenging for academics and the industry to
seamlessly achieve interoperability among blockchains.
  Given the novelty and potential of this new domain, we conduct a literature
review on blockchain interoperability, by collecting 262 papers, and 70 grey
literature documents, constituting a corpus of 332 documents. From those 332
documents, we systematically analyzed and discussed 80 documents, including
both peer-reviewed papers and grey literature.
  Our review classifies studies in three categories: Cryptocurrency-directed
interoperability approaches, Blockchain Engines, and Blockchain Connectors.
Each category is further divided into sub-categories based on defined criteria.
We discuss not only studies within each category and subcategory but also
across categories, providing a holistic overview of blockchain
interoperability, paving the way for systematic research in this domain. Our
findings show that blockchain interoperability has a much broader spectrum than
cryptocurrencies.
  The present survey leverages an interesting approach: we systematically
contacted the authors of grey literature papers and industry solutions to
obtain an updated view of their work. Finally, this paper discusses supporting
technologies, standards, use cases, open challenges, and provides several
future research directions.
