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This paper addresses the optimization of prism-coupled, wavelength-interrogated surface-plasmon resonance (SPR) sensors. Limit of detection (LOD), rather than sensitivity, is chosen as a figure of merit. The Cramer-Rao bound (CRB) for spectral shift estimation allows lower bounds to be placed on LOD independent of interrogation system and estimation algorithm. Analytic evaluation of the CRB for a Lorentzian spectral feature justifies common observations about the relationship between resonance line shape and sensor performance. Numerical evaluation of standard SPR sensor configurations shows that maximizing sensitivity is an insufficient criterion for minimizing limit of detection. Designs optimized for LOD are presented for common SPR sensor materials and geometries.