This thesis investigates the chemistry of graphene from its basic synthesis to further modulation of its structure, geometry and surface chemical functional groups. A series of wet chemistry and dry chemistry experiments were performed. The wet chemistry includes the diazonium salt functionalization, graphene oxidation and reduction, nanotube unzipping chemistry, graphite intercalation and exfoliation. The dry chemistry includes chemical vapor deposition and solid carbon source synthesis of graphene, the control of domain size and stacking order, graphene hydrogenation and lithographically patterned graphene superlattices. With all these chemical approaches, graphene's electrical and optical properties, solubility in organic solvents, crystallography, and chemical reactivity were carefully investigated and discussed. In addition to the fundamental chemistry of graphene, the bio- and environmental impact of this new material was also taken into consideration and investigated.