Numerical simulations of galaxies are an important tool for studying the evolution of the interstellar medium (ISM), and I will start by reviewing some fo the results on star formation and giant molecular clouds (GMCs) which have resulted from these types of simulations. I will also highlight some recent results, which model the nearby galaxy M33, and are able to provide a good match to both the overall structure of M33, and the properties of GMCs observed in CO surveys of M33. However whole galaxy simulations typically lack the resolution to effectively follow cluster evolution, and accurately model processes such as stellar feedback. I will show some current work in our group, which aims to model these processes in more detail. We show simulations which fully model ionisation from massive stars as well as including spiral arms in a galaxy subsection. We find ionisation has a surprisingly large scale effect on the ISM, effecting the densities of filaments and structures along the spiral arms, and triggering new star formation. Such results have not been shown before since previous simulations were typically limited to individual, isolated molecular clouds. We also show more localised simulations which focus on the formation of young massive clusters (YMCs). We propose that the formation of clusters by colliding flows can enable the rapid build up of large amounts of gas over short timescales, matching the observed characteristics of YMCs. We compare some of our simulated clusters to the masses and radii of Milky Way clusters. Finally we highlight future work aiming to fully resolve and model stellar clusters on spiral arm scale simulations, investigating the effects of spiral arm strength, gas velocity fields and stellar feedback.