One of the main existing categories of population structure is related to ethnic, linguistic and confessional aspects. Most countries and regions of the world include the ethno-cultural characteristics of the population in their censuses. The only complex and official source of knowledge of the number and regional distribution of declared ethnicities, languages and religions are population censuses, normally and preferably to be conducted every ten years. The ethnic, linguistic and religious specifics are of particular interest in the analysis of the censuses, being necessary for knowing the realities related to the composition of the population at the national, regional and local level. In the territory of the Republic of Moldova, the ethnic category was included in all censuses, regardless of the period and the existing administration (Russian Empire 1897, Romania 1930, USSR 1926 - 1989, Republic of Moldova 2004, 2014, as well as in the planned census in 2024). The censuses represent the only complete and official source for evaluating the ethnic structure of a state or region, being the reference source both for national and international institutions and organizations, as well as for numerous researches in the field. Ethnicity is a personal and collective identity assumed on the basis of characteristics related to origin, language, culture, tradition, history, religion, race and other aspects, assumed on a subjective level by everyone. Ethnic identity is freely expressed and based on the self-determination of each person, it may or may not coincide with other identities, such as linguistic, religious, origin, region, country, etc. For the first time for the Republic of Moldova, it is proposed to include the declaration of the secondary ethnic identity, in case it is declared by the person. In order to reflect as correctly and complexly as possible the ethnic realities in the population and housing censuses of the Republic of Moldova, a well-prepared and documented tool is necessary, represented by the Classifier / Nomenclator of Ethnicities. It is based on the existing and potential ethnic categories, which emerged as a result of ethnic, historical, political processes and population migration in different periods. The nomenclature of the ethnicities of the Republic of Moldova existing until now, including those officially adopted, such as the Classifier of nationalities and ethnicities from 2001 and the work tools used for the classification of ethnicities at the National Bureau of Statistics, the Civil Status Service, the Bureau of Inter-Ethnic Relations, have not benefited from a specialized expertise and presents numerous errors, both of the categories and of the form and norms of writing ethnicities, being also incomplete. The characteristics related to the linguistic structure were also always present in the population censuses and other officially collected data on the territory of the current Republic of Moldova. In this case, several categories are taken into account: mother tongue, language usually spoken and knowledge of other languages. In various formats, these characteristics were also present in previous censuses, such as mother tongue and knowledge of Russian or other languages in the case of Soviet censuses or mother tongue, language usually spoken and knowledge of other languages in the last censuses of the Republic of Moldova. The situation is more complicated in the case of the confessional structure, the continuity of this characteristic, present in the census of the Russian Empire (1897) and in that of Romania (1930) was disrupted by the Soviet anti-religious policy, which omitted this element in the soviet censuses until 1989. The censuses of the Republic of Moldova from 2004 and 2014 resumed the declaration of religion. As a result of detailed research based on the history and experience of the previous censuses carried out on the territory of the Republic of Moldova, the classifications on ethnicities, languages and religions were made, which will be useful for the next censuses of the population in the Republic of Moldova.