The adoption of lean principles has become a critical focus for organisations striving to enhance operational excellence and competitive advantage. However, measuring the extent of lean implementation, referred to as “leanness”, remains a complex task requiring systematic evaluation methods. This article examines five widely recognised models developed to assess and guide lean maturity: Rapid Plant Assessment (RPA), the Shingo Model, Kobayashi’s 20 Keys, Schonberger’s 16 Principles, and Bicheno and Holweg’s 25 Principles. Each model is analysed based on its primary focus, purpose, suitability, required resources, strengths, and weaknesses. The comparative analysis reveals significant variation among the models: some, such as RPA, emphasise rapid visual diagnosis with minimal resource investment, while others, like the Shingo Model, require profound cultural and behavioural transformation. There is no universal solution; the choice of model should be guided by factors such as company size, industry, lean maturity, available resources, and whether the goal is assessment or improvement. The findings highlight the value of integrating elements from multiple models to achieve a balanced and context-specific lean transformation. Future research should explore empirical applications and combinations of these frameworks to further enhance lean assessment practices.