Recently, Lean Production (Lean) has become a prevailing management concept in Sweden. However, previous research seems to show that the Lean concept and the impact of Lean vary considerably between organizations. This paper illustrates some key methodological issues that need to be considered when researching loosely defined management concepts such as Lean. The paper is based on a review of the literature and five comparative Swedish cases studies. Our study indicates that Lean has changed over time and that operationalization and interpretations of the concept vary considerably. This study concludes that future Lean studies should include a thorough assessment of the Lean interventions, study settings, and in particular non-Lean factors mediating the outcomes of Lean-inspired change programs.