Manufacturing competitiveness highly depends on companies' ability to rapidly reconfigure their assembly systems. This paper introduces the concept of assembly system proactivity based on interrelated levels of human involvement in a planed way will contribute to increased system ability to proactively address predicted and unpredicted events. Correct involvement of human operators will utilize the full combined potential of human and technical capabilities, also providing cost-efficient assembly system solutions. The ProAct (project presented) will develop proactive assembly system models, evaluating proactive, feature-based solutions. Focus is on realizing the potential of semi-automated system with relevant human involvement, i.e. operators with high skills adding e.g. flexible capability and functionality.