The book contains 14 essays on international planned languages, known as universal languages, world auxiliary languages, and constructed languages, outlining the currently most important ones in their role as means of communication – primarily Esperanto, but also Ido and Interlingua. It introduces the subject of Interlinguistics, points to relevant literature, and discusses the role of planned languages in the works of Wilhelm Ostwald, Eugen Wüster, and Ernst Beermann. Contributions to lexicology, morphology, and lexicography (typology of dictionaries), as well as to specialized communication and translation of planned languages, provide insights into their structure, development, and function. Esperanto is at the center of the book due to its widespread use and scientific description, known as Esperantology.