The structure of the Sgr B2 molecular cloud has been studied by detailed and extensive mappings of the CO, ^(13)CO, CS, and H_2CO (2-cm) transitions. The cloud consists of a dense core at the OH maser position plus a very large envelope 45 pc across. Sagittarius B2 is one of the most massive objects in the Galaxy, containing about 3 x 10^6 M_⊙ - an estimate arrived at independently from calculated ^(13)CO column densities, from application of the virial theorem, and from far-infrared measurements. The molecular gas has an average temperature T_k of ~ 20 K; that this is similar to the temperature of grains emitting far-infrared radiation is taken as evidence of near thermal equilibrium between dust grains and H_2. The dynamics of the gas in the cloud is found to be dominated by large-scale systematic motions with velocity not a monotonic function of radius
