The recently proposed metabolic theory of ecology (MTE) claims to provide a mechanistic explanation for long known allometric relationships between mass and metabolic rate. The MTE postulates that these patterns of allometry are driven by the primary selective constraint of transport of energy and materials. However, recent evidence along several different lines has called into question both the adequacy and the universality of this mechanism. We review the accumulating body of literature on this subject, adding our own concerns and criticisms. In addition to other difficulties, we argue that MTE fails as a mechanistic explanation of mass versus metabolic rate allometries because: 1) circulatory cost minimization is not a tenable criterion for evolutionary optimization, 2) the Boltzmann type relationships on which MTE depends are inadequate descriptors of complex metabolic pathways, and 3) most of the hypotheses advanced by the MTE do not, in fact, depend on the proposed mechanism and therefore cannot be used to test the theory. We conclude that the MTE should be abandoned as a monolithic explanation for allometric patterns, and that a more realistic path toward a better understanding of allometry would be to consider multiple explanatory mechanisms for physiological allometries.Biologists have been intrigued by allometric scaling relationships between organismal form, function, and ecology for more than a century. Such scaling relationships are taxonomically pervasive and have inspired a literature that, by 20 years ago, resulted in three nearly simultaneous, but remarkably different, book-length reviews (Peters 1983, Calder 1984, Schmidt-Nielsen 1984. Nonetheless, there is currently no widely accepted general mechanism for these patterns. Hypothesized explanations include structural and functional factors (e.g. surface area/volume effects on exchanges of heat and metabolites; reviewed by Calder 1984, Dodds et al. 2001, biomechanical requirements for support and fracture resistance (McMahon 1973, Biewener 2005, and correlated natural selection on body size and life history (Kozlowski and Weiner 1997).Recently, West et al. and others have advanced a metabolic theory of ecology (MTE Á West et al. 1997, 2002, Brown et al. 2004a which:1. conjectures that a quasi-fractal structure of distributional networks like circulatory systems requires an allometry (e.g. metabolism 0/a mass b , where a is a ''normalization constant'' and b is a scaling exponent) in order to minimize transport costs of energy and materials 2. argues that minimizing these transport costs requires a scaling exponent of b 0/3/4, which in