The aim of this book is to study how application programming interfaces (APIs) can be protected by intellectual property rights. Primarily, it is examined how, and to what extent, copyright protection can be applied to APIs in the light of European and U.S legislation and case law. The book also studies how patents can be used for protecting APIs and what consequences patent protection of APIs may cause. The research is a legal dogmatic study, including also comparative elements between different jurisdictions as well as practical examples. Many companies have recently realized the value of open API information in highly competitive market environment. The recent development of legal environment, including the outcome of Microsoft case in Europe, is promoting creation of interoperable software and hardware products as well as technical development which avails the whole society. The book should be helpful for professionals working in the fields of IPRs and software development.