This paper provides summary judgments about the strength of the evidence that iron deficiency or anemia causes the six functional outcomes that were reviewed in the preceding papers, highlights priority research needs and discusses the implications of these conclusions for advocacy and programs. A significant body of evidence supports a causal relationship between iron deficiency and deficits in work productivity and child development, and between severe anemia and maternal and child mortality. Causal evidence is lacking or contradictory for iron deficiency and low birth weight and infectious disease, and for mild-to-moderate anemia and child or maternal mortality. Future research is required to expand or refine these conclusions. In the meantime, advocacy should focus on the outcomes supported by substantial causal evidence. These causal relationships can be separated into a problem of severe anemia (which causes mortality) and tissue iron deficiency (which causes deficits in work performance and child development). The simple model depicting severe anemia and tissue iron deficiency as distinct but overlapping entities with distinct functional outcomes is presented and may be useful in designing and evaluating programs.