This article is offered in two parts. In the first part, after reviewing the concepts and definitions of the household on the basis of statistical sources, an attempt is made to elaborate on the effect of changes in definitions on the number and mean size of the household and to study the basic characteristics of household in Iran by taking those changes into the consideration. Among the characteristic studied are the number and the type of household, growth rate of households in comparison to the growth rate of total population and the structure of household by size and head of household's characteristics. Inconsistency in the growth rate of household compared with total population between 1991 and 1996 which raises the question of either under enumeration or incorrect coding and tabulation of 1996 census is an important finding of the first part of the article. In addition to that, reduction in the share of households with large numbers and, from there, reduction in the mean size of the household, increase in the household headed by a woman with no employed member and increase in the importance of private sector activities as the main source of the head of the household's employment are among the salient findings of this part. In the second part emphasize is made on the inefficiency of the current methods for projection of the number and mean size of the household which is mainly due to the reduction of fertility after 1986. By evaluation of current methods, such as the regression method and the method of incorporating changes in the size of the household with future changes in fertility, an attempt is made to project the number and size of the household in the next few decades.