The early evolution of dense stellar systems is dominated by the majority mass component – the gas – and so any credible modeling of the first Myr or so of a cluster's life inevitably involves hydrodynamical simulations. Such simulations have increased considerably in sophistication over the last few years and are now beginning to incorporate the effects of stellar feedback, thus enabling one, for the first time, to model the formation of populous clusters. In this review I focus on two issues that have arisen from the simulations – the relationship between maximum stellar mass and cluster mass, and the issue of the maximum density that is attainable during the cluster formation process. I also report on the first results of new simulations that model feedback from ionising radiation.
