Diversity of viewpoint is discussed as a major determinant of system complexity – in particular as affecting project complexity during design, development, manufacture and roll-out. The viewpoints of particular interest are those arising from diversity of implementation technology, and from multiple specialist engineering disciplines. The paper discusses the historical treatment of complexity in design offices, the challenge of diversity to a project, and its impact on the formal and informal organisation of the project, and on individual cognition. Conclusions are drawn for system acquisition and for design and development approaches. Metrics for the challenge posed by the diversity of a project are needed, so that organisational capability can be aligned with project need – for project processes in particular. Complexityas-diversity makes demands on the organisational climate and the treatment of multiple (potentially conficting) sources of bad news. Interactive visualisation is considered to reduce cognitive demands, support individual and team decision making, and to have an effect equivalent to reducing complexity.