A quick look at the diffuse interstellar medium.- Radiative transfer considerations.- The diffuse ISM from the ground - chemistry and tracers.- Observing the diffuse ISM - making sense of the radio observations.- Observing the diffuse ISM - the space missions.- Observing in the Dark: The Dust-Gas Connection.- Surveying the molecular Milky Way.- The relationship between CO and H2.- Surveys for high-latitude molecular clouds.- Distances.- Dynamical...