What is blood? The many meanings of blood vividly attest to its polyvalent qualities and its unusual capacity for accruing layers of symbolic resonance. Life and death, nurturance and violence, connection and exclusion, kinship and sacrifice - the associations multiply, flowing between domains in a quite uncontainable manner. Whether expressed in the rhetoric of familial, racial, ethnic, or national exclusion, or in calls to violent action, idioms of blood often have exceptional emotional force. Blood has the capacity to flow in many directions: it is literally present in spaces of blood donation, and metaphorically central to sanguinary idioms in depictions of the economy. These essays illuminate through close anthropological and historical scrutiny blood's special qualities as bodily substance, material, and metaphor. They suggest many reasons for elucidating a theory of blood.