The aim of this thesis is to describe of the chat application in the virtual reference services from both theoretical and practical point of view. The theoretical part is based on close reading of the mostly foreign literature. The first introductory chapter discusses the terminology and definition of reference services and then virtual reference services, which have not been settled yet. The second chapter deals with chat application. All phases of the chat application are mentioned, from planning through implementation and marketing to the final evaluation of the service. The third chapter is devoted to chat as a specific form of communication with particular focus on its advantages and disadvantages in the library services. The findings of these chapters are used in the analysis of the chosen virtual reference services. The practical part in the fourth chapter constitutes chat in real use, first in foreign libraries, then in Czech libraries as well. The fifth chapter constitutes the main contribution of the practical part. It describes the chat preparation within the Czech cooperative virtual reference service "Ask your library" in the National Library of the Czech Republic, in which the autor of this thesis participated. The conclusion provides a brief summary.