In developing countries, population estimates and assessments of progress towards the Millennium Development Goals are based increasingly on household surveys. It is not recognised that they are inappropriate for obtaining information about the poorest of the poor. Typically, they omit "by design:" those not in households because they are homeless; those who are in institutions; and mobile, nomadic or pastoralist populations. In addition, "in practice," because they are difficult to reach, household surveys will typically under-represent: those in fragile, disjointed or multiple occupancy households; those in urban slums; and may omit certain areas of a country because of security considerations. This article reviews estimates of the numbers of children in each sub-group, focussing on countries endorsed by the Fast Track Initiative, and estimates that 21 million rather than 14 million children are out-of-school in those countries; and that worldwide 115 million rather than 67.5 million are out-of-school. Methods of improving population counts are briefly discussed. (Contains 5 tables and 4 notes.)