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. Drawing together the historical and ethnographic case studies presented in this volume – from the literal presence of blood in spaces of blood donation to the metaphorical deployment of sanguinary idioms in depictions of the economy – this introduction examines blood's special qualities as bodily substance, material, and metaphor. In sketching out a ‘theory of blood’, it suggests why such a comparative undertaking might be of value.