The mechanism of spiral structure in galaxies is a puzzle that is only partly understood. Galaxies do not revolve like solid bodies, so the spiral patterns cannot be entirely material in nature. Yet the observable tracers of spiral structure are unquestionably material: young stars, dust and gas. These objects must be organized by a collective, wave phenomenon, the spiral density wave in the disk star population. The density wave is a small (≈ 10%) local increase in the density of stars. The interstellar medium responds to the increased gravitational force by forming Giant Molecular Clouds (GMCs), concentrations of 105 M⊙ or more of interstellar matter in a region about 50 pc across. In a sense the GMCs are the spiral arms: in other galaxies, dust, gas and young star populations trace spiral structure; in the solar vicinity, these populations are seen to be associated with GMCs. This paper briefly reviews observational data supporting the hypothesis that spiral structure results from the agglomerative build-up of GMCs from smaller clouds, that this growth occurs preferentially in spiral arms, and that GMCs subsequently self-destruct because of the formation of massive stars.