Galactic star formation is surprisingly inefficient. Yet, recent large-scale surveys of dark clouds in the Galaxy show that one rarely finds molecular clouds without young stellar objects. Thus, star formation must occur rapidly upon molecular cloud formation. This rapid onset of star formation puts strong constraints on the physical properties of the parental cloud, and it mandates that a cloud's structure and dynamics controlling stellar birth must arise during its formation. Hence, in order to understand how stars form it is crucial to understand how molecular clouds form. I will outline the physics of flow-driven molecular cloud formation, identifying the dominant processes which lead to both the observed cloud structure and the rapid onset of star formation. I will discuss the relevance and the limitations of this scenario for setting the Galactic star formation efficiency.