Much has been written and researched on the role of top management in Lean transformations. However, even with top management commitment companies can struggle to keep up kaizen activities once the so-called implementation period has passed. Little has been made of the role of the middle managers. Often overlooked and underappreciated, the development of middle managers leadership skills through gemba based training and hansei (self-reflection), might be the missing link in successful and sustainable lean transformations. Based on a literature review on the subject and case studies of three different companies, we present how middle managers can become an integral part in sustainable lean transformations by taking the role of trainers, and leading on-the-job development of both technical-, improvement- and teamwork skills. This should be done by teaching Toyota Production System (TPS) on the gemba, creating space for Hansei. The training should be carried out in a train-the-trainer system where each manager is responsible for the training and development of their direct reports.