Starting from census data on co-residence and household composition, the authors analyse principles of family organisation and family formation in twentieth-century urban Russia and the Soviet Union. The article uses an adapted version of the classification of households developed by Peter Laslett and Eugene Hammel to study variation in household structure for successive population censuses. Changes in this variation between cross-sections are explained with the help of additional quantitative and qualitative data and are linked to the fundamental demographic, social and economic shifts which took place in Russian society in the course of the twentieth century. The article finds a family system characterised by a tendency towards nuclear family formation, but incorporating a fairly stable element of household extension. Co-residence of three generations was both an answer to a perennial housing problem and offered important advantages in the sphere of childcare and care for the elderly. Variation and fluctuation in household structure are found to be most pronounced during the turbulent first half of the century. After a period of stability during the post-war decades of Soviet rule, post-Soviet transformations provoke new changes.