Bicyclic and related strained olefins were studied by the ab initio/GIAO-CCSD(T) method. Structures and C-13 NMR chemical shifts of the olefins were calculated using ab initio/GIAO-CCSD(T) method. The delta C-13 of the olefinic carbons of the yet unknown bicyclo[1.1.0]but-1,3-ene 1 and bicyclo[ 2.1.0] pent-1(4)-ene 2 were computed to be 69.4 and 212.4, respectively, at the GIAO-CCSD(T)/qzp/tzp//MP2/cc-pVTZ level. The delta C-13 of the olefinic carbons of the intriguing (larger and also yet unknown) tricyclo[3.3.1.0(3,7)] non- 3(7)-ene 6 and cubene 7 were calculated to be 172.5 and 187.4, respectively, at the GIAO-CCSD(T)/tzp/dz// MP2/cc-pVTZ level. In a related study, the relative energies of the various conformers of ethylene were computed and were found to correlate extremely well with the 13C NMR chemical shifts, reflecting the linear dependence of the C-13 NMR chemical shifts on the internal strain of the molecules.