"Starting in the 1980s with the first Toyota controlled production plant in the United States (NUMMI), the idea of Lean as derived from the Toyota Production System has become a popular method to improve production processes as well as any other procedure in organizations around the world. Over the last three decades a multitude of literature on Lean implementation and consulting companies offering help with the Lean transformation have emerged. The success rate of real Lean transformations however is often estimated to lie below 10% by experts such as Clifford Ransom or members of the Society of Manufacturing Engineers (SME). This leads to the question of why most companies fail to sustain the Lean improvements that most of them are able to initiate during the early implementation stages. This thesis will therefore investigate the root causes that lead to a time dependent loss of Lean improvements from three different perspectives. One standpoint is given by the TPS beliefs which are united in the House of Lean. The second angle on the topic is drawn from an online search of expert opinions and the final evaluation is taken from analyzing a real case scenario at a plant in Trumbull, CT. After this thorough analysis of possible root causes a set of countermeasures is presented that will support an organization in avoiding the major mistakes of a Lean implementation. The three countermeasures include standardization to build a base for Lean improvements, employee involvement to assure sustainability through conviction and continuous instead of rapid improvement. Every countermeasure will be described theoretically as well as with the help of examples from real life scenarios to enable the reader to apply the suggestions immediately. A final discussion will then examine the improvements that where reached at the production plant in Trumbull, CT and show how the combination of all three countermeasures ensures the sustainability of these improvements."