The concepts of the digital, virtual and electronic library are often used as synonyms. The digital (electronic) library is the systematic collection of identifiers pointing to documents stored elsewhere, available via networks. The library of the future will not consist of digital documents only. Paper and digital documents will form the "hybrid" library together, a concept introduced first in England in the ELIB library project. Questions rising in hybrid libraries (such as copyright issues, longer readability, document protection) are discussed by the article.