The tetramethyl and tetra-tert-butyl derivatives of cyclobutadiene and tetrahedrane have been studied with ab initio and density functional methods. The ring in tetra-tert-butylcyclobutadiene displays very unequal bond lengths (1.354, 1.608 Angstrom) and confirms the earlier suspicion that the low-temperature X-ray structure was distorted. The C-C single bonds have the longest separations found to date between sp(2)-hybridized carbons. Tetra-rert-butyltetrahedrane, which prefers T over Td symmetry, is calculated to be 1.5 kcal/mol more stable than tetra-tert-butylcyclobutadiene (B3LYP/6-311+G(d)//B3LYP/6-3 1G(d)). The dications, C4R42+, dianions, C4R42-, and dilithiated species, Li2C4R4 (R = H, Nle, t-Bu), also were studied to determine the effect of substitution on structure and stability. Finally, NICS (nucleus-independent chemical shifts) values were calculated and showed, as expected, the dications and dianions (4n + 2 pi-electrons) to be aromatic (negative NICS(0)) and the cyclobutadienes to be antiaromatic (positive NICS(0)).