Citations are part of publishing in science, and scientific areas differ in use of citations. Mostly, only previously published papers which had been useful in writing and publishing the new work are cited. Therefore, the number of citations points to effective presence of a paper or an author in a scientific community, and it is used in the evaluation of scientific impact of an author, institution, discipline, country etc. Sources of data for so called citation analyses are citation indexes. Results of citation analyses should be assessed with caution, in the context of other indicators of scientific impact.