In developing countries, assessments of progress toward development goals are based increasingly on household surveys. These are inappropriate for obtaining information about the poorest. Typically, they omit by design: the homeless; those in institutions; and mobile, nomadic, or pastoralist populations. Moreover, in practice, household surveys typically under-represent: those in fragile, disjointed households; slum populations and areas posing security risks. Those six sub-groups constitute a large fraction of the “poorest of the poor”. We estimate that 250 million are missed worldwide from the sampling frames of such surveys and from many censuses and their omission may well lead to substantial biases.