The development of information technology enabled the development of a new form of library, the so-called e-libraries. It is certain that they will never completely replace physical libraries, but every day their importance is increasing. The basic features of e-libraries are identical to physical libraries, but the former are more focused on providing support to individual needs with a series of specific applications. Also, the digitization process of some materials ensured their preservation from destruction, although digitized materials require continuous information maintenance in order not to become obsolete. In order for e-libraries to be able to network with each other, the features of the information systems, as well as the permitted formats, are prescribed. Examples of e-libraries are discussed in the paper: the Library of Congress of the United States of America, the National and University Libraries in Zagreb and the Baltazar Zaprešić University Library. These e-libraries differ significantly in terms of the amount of material and the number of users. The analysis of their information systems showed that the complexity of the system is proportional to the importance of the library itself. Virtual environments are also significantly different in terms of importance and investments. However, in all three cases, the basic roles of the e-library have been fulfilled, namely the ease of searching and the availability of available materials.