Family income is an important factor associated with children's educational achievement. However, key areas of UK research (for example, on socially segregated schooling) and policy (for example, the allocation of funding to schools) rely on children's free school meal (FSM) "eligibility" to proxy family income. This article examines the relationship between children's FSM "eligibility" and equivalent net household income in a nationally representative survey of England (the Family Resources Survey). It finds that children "eligible" for FSM are much more likely than other children to be in the lowest income households. However, only around one-quarter to one-half of them were in the lowest income households in 2004/5. This is principally because the receipt of means-tested benefits (and tax credits) pushes children eligible for FSM up the household income distribution. The implications for key areas of research and policy are discussed. (Contains 8 notes, 5 tables, and 1 figure.)