The work described in this paper forms part of CONTESSB, a collaborative project performed by a consortium of major companies and universities in the United Kingdom. The project concerns the use of environmental simulation in the testing, evaluation, and validation of high-integrity systems in a range of industrial areas including the nuclear, defence, aerospace, avionics, automotive, railway, and process control industries. The paper looks at the requirements for environmental simulators for critical systems and discusses their use in demonstrating system safety. It also considers the development of the simulator and suggests methods of integrating this into the overall project development lifecycle.