1. Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA
2. Department of Pathology, Northwestern University Medical School, Chicago, Illinois 60611, USA

Correspondence to: Z. N. Oltvai²A.-L. Barabási¹ Correspondence and requests for materials should be addressed to A.-L.B. (e-mail: Email: alb@nd.edu) or Z.N.O. (e-mail: Email:  zno008@northwestern.edu).

In a cell or microorganism, the processes that generate mass, energy, information transfer and cell-fate specification are seamlessly integrated through a complex network of cellular constituents and reactions¹. However, despite the key role of these networks in sustaining cellular functions, their large-scale structure is essentially unknown. Here we present a systematic comparative mathematical analysis of the metabolic networks of 43 organisms representing all three domains of life. We show that, despite significant variation in their individual constituents and pathways, these metabolic networks have the same topological scaling properties and show striking similarities to the inherent organization of complex non-biological systems². This may indicate that metabolic organization is not only identical for all living organisms, but also complies with the design principles of robust and error-tolerant scale-free networks^{2, 3, 4, 5}, and may represent a common blueprint for the large-scale organization of interactions among all cellular constituents.