We review recent developments on the electronic properties of graphene under the influence of adsorbates. Potassium and hydrogen adsorbed on graphene induce very different effects on the graphene electron gas because of the different types---ionic versus covalent---of chemical bonds formed. Potassium readily donates electrons to graphene, and the resulting Fermi sea shows strong electron-plasmon scattering but weak defect scattering. In contrast, hydrogen adsorption saturates a carbon pi bond, leading to the removal of electrons from the graphene. Such hydrogen bonds act as a lattice defect, leading to a sharp reduction in conductivity and an insulating temperature dependence of the resistivity. The marked contrast in the behaviour of these adsorbates derives from the different symmetry classes of their defect geometries.