For many years, lean production has been successfully applied in large companies producing high volumes of standardized products. However, companies which operate in dissimilar environments have yet to expose a suitable model for pursuing the lean ideal, adapted and fine-tuned to the diverse characteristics demonstrated by producers of, for example, highly customized, engineer-to-order products. The aim of this paper is to examine the evolution of lean principles with the primary goal of converging towards a new set of principles that are more clearly aligned for the deployment of lean in engineer-to-order manufacturers. We take insight in lean production, lean project management, and lean product development in order to develop a set of principles which we suggest is more clearly suited for the deployment of lean thinking in engineer-to- order manufacturers. Firstly, we use literature review in order to examine the most prevalent lean principles in the extant literature, and we apply qualitative content analysis in order to propose a new set of principles. We then adopt a multiple-case study approach in order to validate the derivation of the new principles in the context of two, distinct engineer-to-order environments. Our findings highlight a transition from the traditional lean production model to a more contemporary, innovative approach for pursuing the lean ideal in the context of ETO manufacturers.