The intrinsic secondary structure-forming propensities of the naturally occurring amino acids have been measured both experimentally in host–guest studies and statistically by examination of the protein structure databank. There has been significant progress in understanding the origins of intrinsic α-helical propensities, but a unifying theme for understanding intrinsic β-sheet propensities has remained elusive. To this end, we modeled dipeptides by using a van der Waals energy function and derived Ramachandran plots for each of the amino acids. These data were used to determine the entropy and Helmholtz free energy of placing each amino acid in the β-sheet region of φ—ψ space. We quantitatively establish that the dominant cause of intrinsic β-sheet propensity is the avoidance of steric clashes between an amino acid side chain and its local backbone. Standard implementations of coulombic and solvation effects are seen to be less important.