Molecular gas and star formation in the Milky Way

The dense molecular gas is the ideal tracer of the spiral structure in the Milky Way, and should be used intensively to solve the puzzle of its structure. In spite of our position inside the plane, we can hope to disentangle the structures, with position-velocity diagrams, in addition to $(l-b)$. I summarize the state of the art simulations of gas flows in the MW, and describe what can be done to improve the models, taking into account the star formation, in view of what is already done in external galaxies, with a more favorable viewing angle.