||Industry 4.0 brings a high degree of flexibility to the production process through the use of self-optimizing cyber-physical systems. Deterministic planning is, therefore, not possible. Planners, workers and machines will have to be more flexible and adapt quickly to changes in demand. In this context, planning software will become an indispensable tool and a decisive factor for economic success in the production process. A more flexible production process requires deep and comprehensive operational knowledge as well as software that helps users cope with the hardly manageable functional diversity of CAx systems and the requirements of just-in-time demand. This software needs to be developed. CAx systems are highly complex, and operating mistakes affect the entire value-added chain. Software development is primarily functionally driven. The result is highly complex systems that require expert knowledge to operate. Work patterns, the working context and user profiles constantly change. The increasing number of non-standard tasks is growing, and this requires comprehensive operational knowledge. At the same time, the number of experienced users is decreasing. Demography and workers with diverse backgrounds (migration) require low-threshold software solutions with less complex interfaces. Lean-CAx is the next generation of CAx systems with “lean” interfaces that enable users to cope with the technological complexity: from providing primary functionality to user-oriented expert systems. This results in commercially strong CAx systems for Industry 4.0 characterized by: lean interfaces; verified, transferable engineering methodology; tools for evaluating and optimizing the interfaces of CAx systems.