Graphene appears to be an excellent candidate for spintronics due to the low spin-orbit coupling in carbon, the twodimensional nature of the graphene sheet, and the high electron mobility. However, recent experiments by Tombros et al. [Nature 448, 571 (2007).] found a prohibitively short spindecoherence time in graphene. We present a comprehensive theory of spin decoherence in graphene including intrinsic, Bychkov-Rashba, and ripple related spin-orbit coupling. We find that the available experimental data can be explained by an intrinsic spin-orbit coupling which is orders of magnitude larger than predicted in first principles calculations. We show that comparably large values are present for structurally and electronically similar systems, MgB 2 and Li intercalated graphite. The spin-relaxation in graphene is neither due to the Elliott-Yafet nor due to the Dyakonov-Perel mechanism but a smooth crossover between the two regimes occurs near the Dirac point as a function of the chemical potential.