Accurate mass measurements of galaxy clusters are a vital tool in modern cosmology, providing a unique insight into the nonlinear regime. There has been a wealth of work characterising the bias in X-ray and weak lensing mass measurements using numerical simulations, however these have been restricted to clusters with masses less than 10^15 solar masses or dark matter only simulations. The MACSIS project is a set of hydrodynamical simulations of massive galaxy clusters which extends the BAHAMAS sample of galaxy groups and clusters to higher masses. These simulations use the sub-grid physics models developed in the BAHAMAS project, which includes feedback from supernovae and active galactic nuclei. Taken with the BAHAMAS sample, the simulations form a cluster population which covers almost two orders of magnitude in mass, with more than 250 clusters with masses greater than 10ˆ15 solar masses at z=0. Since these simulations consist of both dark matter only and hydrodynamic simulations, they are ideal for investigating the impact of baryons on cluster weak lensing. Together with the calculated X-ray observables of MACSIS clusters, these are used to infer the hydrostatic mass bias for massive galaxy clusters and its dependence on cluster properties. In this talk I will begin by discussing how baryons affect the weak lensing shear profiles of massive galaxy clusters and the implications of this for cluster mass measurements. I will conclude by presenting the hydrostatic mass bias as found in the MACSIS simulations in the context of recent observations.