PurposeLean remains popular in a wide range of private and public sectors and continues to attract a significant amount of research. However, most of this research is not grounded in theory. This paper presents and discusses different expert viewpoints on the role of theory in lean research and practice and provides guidelines for future research.Design/methodology/approachSeven experienced lean authors independently provide their views to the question “is Lean a theory?” before Rachna Shah summarizes the viewpoints and provides a holistic outlook for lean research.FindingsAuthors agree, disagree and sometimes agree to disagree. However, a close look reveals agreement on several key points. The paper concludes that Lean is not a theory but has plenty of theoretical underpinnings. Many lean-related theories provide promising opportunities for future research.Originality/valueAs researchers, we are asked to justify our research drawing on “theory,” but what does that mean for a practice-driven phenomenon such as lean? This paper provides answers and directions for future research.