This special report describes the results from the second task of a two-year project to investigate the impact of system and software complexity on aircraft safety and certification for the Federal Aviation Administration. The first task focused on a literature review of what is known about complexity, definitions of complexity, and the causes and impacts of complexity. This second task focused on identifying candidate measures of complexity for systems with embedded software that relate to safety, assurability, or both. This report begins with the task specification, motivation, and definition of complexity. The second section provides general thoughts about measuring complexity, and then the primary outcome of this research task: the candidate metrics for complexity that could relate to aircraft safety and assurance, including descriptions, sources, and strengths and weaknesses of each measure. Section 3 discusses the relationship of complexity, and complexity measures, to system safety. It reports on conclusions from attempting to analyze impacts of complexity from reported accidents and then from incidents. A brief discussion of consequences of the growth of complexity follows.