Stars are known to form in massive clouds of molecular hydrogen (GMCs), which are highly structured and turbulent. Understanding the origin and evolution of these clouds is a key problem in astrophysics. Here at the LMU and MPE we perform simulations of galaxies and colliding flows to better understand the nature of GMCs and their formation. Simulations of the interstellar medium (ISM) in galaxies show the formation of GMCs by the agglomeration of smaller clouds, as gas passes through the spiral arms (left). These calculations highlight how the properties of the clouds are related to their formation: the spacing of the clouds along the arm is proportional to the strength of the spiral potential, whilst the cloud rotations and internal velocity dispersions are regulated by cloud-cloud interactions and collisions. Without stellar feedback, these clouds last several 10's, or even 100 or more Myrs. We are currently performing calculations which include stellar feedback. In this case, the lifetimes of the GMCs are typically much shorter, the lifetimes and also the morphologies of the clouds governed by the star formation efficiency adopted in the calculations.