Three different conformations of cyclopentasilane, Si5H10, have been analyzed in detail using ab initio and density functional methods at the HF/B2, MP2/B2, CCSD/6-31G*, and B3LYP/B2 levels of theory. While the envelope (C-s) and the twist (C-2) forms have essentially the same energy, the planar form (D-5h) is ca. 1.3-2.9 kcal mol(-1) less stable. The averaged computed Si-Si bond length of 2.356 Angstrom (for C-s and C-2, each) is ca. 0.02 Angstrom larger than the r(e) value of 2.332 Angstrom derived from an electron diffraction study.