Mass customization and more variants, components, and frequent changes increase production complexity. This paper presents research project aimed at developing a feasible definition of complexity, a method for measuring complexity, which supports line rebalancing, man-hour planning, and complexity management, competence, and information support. The project is done in collaboration between Swerea IVF, Chalmers, Volvo Cars, Electrolux, Stoneridge, Electronics, and AB Volvo. Industrial studies were carried out to understand the needs for handling complexity. In parallel, a literature study was conducted over research on definitions, models and methods for complexity. In literature, complexity is commonly modelled by information and entropy of the system (randomness); and categorized into static/dynamic and subjective/objective. The research further concludes: unknown events increase with complexity, making management of uncertainty increasingly important, not only reducing but also coping with complexities; the role of humans and technology in work systems is crucial for coping with uncertainties; complexity models and methods must be easy to understand and use; and must take a holistic view of production; and include different user’s perspective and the subjective complexity.