Formation of molecular clouds and filaments from the ISM
I will start by reviewing various observations which suggest that molecular clouds form out the HI gas. Then, I will review the models of molecular cloud formation by colliding flows which have been recently developed and which directly illustrate these observations. Consequences of this scenario on the molecular cloud structure will be emphasized and qualitatively compared with observations. Finally, I will present various mechanisms which could explain the formation of filaments within molecular clouds. I will also discuss in which conditions they could work and how to possibly discriminate between the various possibilities.