This special report describes the results from the first task of a two-year project for the Federal Aviation Administration to investigate the impact of system and software complexity on aircraft safety and certification. The first task was a literature review sampling what is known about complexity. The term complexity is often used but not often defined, especially in the context of safety and assurance; our review had to intuit what different sources intended as the meaning by how they used the term in their writings. Essentially, sources described a number of different causes and impacts of complexity. The causes and impacts as described in the literature are presented here in a hierarchy of categories. To use the word “complexity” precisely, the causes and the impacts should be specified. Some sources also discussed measurement and mitigation of complexity; both measurement and mitigation can apply to both causes and effects.