This chapter is not intended as an exhaustive review of the long-running debate surrounding the definition and measurement of poverty. I have considered the essentials of the debate because of its importance in establishing the continued existence of poverty in developed countries and in giving a new impetus to the exploration of the effects of poverty on health. It also serves to warn the reader of some of the pitfalls inherent in defining and measuring poverty. In the brief review of approaches to measurement I have considered the profusion of measures of poverty, deprivation and socio-economic status used in government and international statistics and in health studies so as to inform subsequent chapters (Part Two) which consider the evidence linking poverty and child health and the causal debate (Part Three).