The goal of this proposal is to revolutionize our understanding of star formation in nearby galaxies, using numerical simulations. Traditionally, research in star formation has considered the contraction of a giant molecular cloud (GMC), or more commonly a star forming core, under gravity. However there has been relatively little research on molecular clouds themselves, even though they provide the initial conditions for star formation, and thus determine the main assumptions for theories of star formation. The proposed research will focus on the scales of giant molecular clouds, the clouds of molecular hydrogen (H2) where most star formation takes place in nearby galaxies. These objects link galactic scale physics with the small scale physics of star formation. Only now are the computational resources becoming available to study the interstellar medium (ISM) numerically on galactic scales, and model the complex processes involved in GMC and star formation. Simultaneously observational programs (e.g. ALMA, Herschel, CARMA) are starting to resolve GMCs in nearby galaxies. Our research will involve performing calculations on scales from individual GMCs to interacting galaxies, and comparing to forthcoming observations to answer some of the most fundamental questions in star formation, such as why star formation is inefficient, how do GMCs form and what are their lifetimes.