The human side of highly automated maritime systems can often be neglected in their development. Paradoxically, history and scientific studies have shown that with highly automated systems, there is still a crucial need for humans to monitor the automated operations. Humans also need to intervene to and control the automated operations particularly in exceptional situations and maintenance operations. Therefore, especially human factors engineering is in a key role when developing maritime autonomous surface ship (MASS) systems. This paper discusses some of the related issues, like automation awareness, cognitive workload, trust in automation and technology acceptance that should be considered in detail when developing MASS solutions. A case study is presented on the development of a ship-handling simulator with an autonomous ship collision avoidance system and it is discussed how to apply the simulator to human factors-oriented studies of MASS systems design and evaluation. The design implications for MASS development on a more general level are also presented. By taking the human aspects of MASS systems as a central focus point in the development, it is possible to create safe and successful maritime innovations for the future.