Among what are now more than 1000 efforts to create an international language, primarily the project of L. L. Zamenhof (1887) has developed into a living and flexible language. Although Zamenhof’s hopes for a language accepted worldwide were not fulfilled, Esperanto proves that in principle it is possible to create a new language through language planning and bring it to practical use. This is an important fact for linguistics. Esperanto’s success also lies in the fact that so far it has been able to resist competition from other systems of planned language. The factors that explain this success relate in part to linguistic structures (e.g. the system’s potential for development) and in part to sociolinguistic and language policy considerations. Of particular significance was Zamenhof’s language-policy role: he saw language as primarily a social phenomenon, he linked humanistic ideals to his language, and he passed Esperanto on to an emerging language community with all of its evolving and varied communicative needs. Zamenhof intuitively understood several important factors that contributed to Esperanto’s stability, for example the need for a standard and its codification. Over the past decades, the scholarly literature of Esperanto studies has grown in quality and is regularly recorded in the bibliography of the Modern Language Association of America (MLA).