Lean Thinking is currently often positioned as the underlying theory of lean production among practitioners and academics, although its originators, Womack and Jones, seem not to have presented it as a theory. This paper endeavours to analyze whether Lean Thinking can be viewed as a theory of lean production. For this purpose, a critical assessment of Lean Thinking is carried out. Lean Thinking is argued to lack an adequate conceptualization of production, which has led to imprecise concepts, such as the term “value”. The five principles of Lean Thinking do not systematically cover value generation, and they do not always encapsulate the core topics in their respective areas. The failure to trace the origin of lean concepts and principles reduces the opportunity to justify and explain them. Despite claims for generality, the application area of the five lean principles is limited to the transformation of mass production, with, for instance, one-of-a-kind production and construction being largely out of scope. It is concluded that it is opportune to move on beyond Lean Thinking, towards a generic theory of production, for acquiring a solid foundation for designing, operating and improving production systems.