The Agricultural Production Systems sIMulator (APSIM) is a widely used simulator of agricultural systems. This package includes functions to create, edit and run APSIM simulations from R. It also includes functions to visualize the results of an APSIM simulation and perform sensitivity/uncertainty analysis of APSIM either via functions in the sensitivity package or by novel emulator-based functions. For more on APSIM including download instructions go to \url{www.apsim.info}.